Before we can discuss whether or not you need a wheel alignment we need to have some basic understanding of wheel alignment angles and how they affect vehicle/tire performance.
Almost everyone sort of understands toe, but toe is not quite as simple as the tires toeing in or out. With a straight front axle and only one tie rod there is basically only what is known as total toe. To understand total toe, it is the sum total of each steer tire's toe amount. Independent front suspensions normally have a multipiece tie rod capable of adjusting each individual side. Adjusting each side is called setting individual toe. Total toe is thus the sum total of these two individual settings on an independent suspension.
Individual toe must be set evenly when the steering is at perfect steer ahead while achieving a perfect total toe amount in doing so. Grossly uneven individual toe can upset a thing called Ackerman effect causing faster than normal tire wear during turns and can even affect stability during turns. Ackerman effect is responsible for slightly toeing the steer tires out only during turns which lend stability to the vehicle during these turns and prevent a scrubbing action on the tire tread. Ackerman effect is machined into the steering components in one fashion or the other causing an action sort of similar to a cam type of action.
The best total toe amount for radial tires is a very small setting making individual toe settings even smaller (approx. 1/2 of the total toe amount per side). Rear wheel driven vehicles normally are toed in meaning the front of the tires are a fraction of an inch closer together than the rear of the tires. Front wheel driven vehicles are often set at zero toe or slightly toed out meaning the front and rear of the tires are even at zero toe and slightly closer at the rear of the tires in the case of them being toed out.
Toe wear can be detected by rubbing your hand sideways across the tire's tread. Sharp edges you feel in one direction but not the other indicates wear. Toe wear will usually be sharp edges on both tires from the inside out or the outside in. Sharp edges that do not follow this rule can be attributed to such things as dogtrotting (thrust angle problems).
Most people are confused by caster, even wheel alignment personnel. While caster somewhat helps the tires return to a straight ahead position after a turn the real effect of caster is at or near a straight ahead course. That's why it somewhat helps the steer tires return to straight ahead (the real thing that makes the steering return is call SAI or steering axis inclination and is built into the steering knuckle or spindle).
Caster is pure and simple load projection. It's function is to lend stability to the steering at the steering wheel. Negative caster causes the steering to be grossly tricky and may well cause shimmy. Positive caster causes the steering wheel to stabilize and makes steering easy to accomplish. Very high caster can also cause steering shimmy but most notably causes one to have to turn the steering wheel farther to negotiate a slight turn plus the steering will be stiffer when turning from the steer ahead position.
Manual steering will need a caster amount of 1/2 to 1 1/2 degrees positive while power steering requires approx. 3-4 degrees positive. That is because manual steering requires greater effort to turn from straight ahead while power steering does not. Power steering will be apt to be tricky if set too low.
The difference in each wheel's amount of caster is known as cross caster which must be kept within 1/2 degree of each other or a drift of sorts will occur. This drift will be toward the side with the lowest caster amount. This is also where you loose a lot of people who fail to understand caster. It is possible to counter the normal right hand drift of larger vehicles using cross caster if the vehicle is equipped with a straight front axle and does not have air ride front suspension. If there is no drifting to the right or left in the front end there will be no whipping of the steering wheel like many claim. There is normally some whipping if there is a drift whether the drift is camber induced or caster induced. The steer tires will last two or three times as long if all drifting is eliminated and handling will be greatly enhanced. Vehicles with independent front suspension can be readily adjusted as far as camber is concerned which also can eliminate right hand drift in most cases.
Now is the time to visit pulling or drifting in the front end. First we need to discuss tire air pressures. Tires that are uneven in pressure (within 5psi or less of each other) will cause a pull but uneven pressures are more complicated when you explore the effects closer. In addition to the added drag of increased surface contact of a low tire, a measurement called scrub radius increases the pull. Scrub radius is the measurement between the intersecting points of two imaginary lines at the ground. One drawn through the pivot points (be it spindles or ball joints, etc.) and the other drawn through the exact center of the tire. Uneven tire sizes on the same axle have the same effect-the pull will be toward the smaller tire because of the increased scrub radius measurement on that side of the vehicle.
Tires of the supposedly same size can pull one way or the other but usually from the time they were new. This is possibly because one tire is shorter or taller than the others upsetting the balance of scrub radius. These tires can be somewhat isolated by simply swapping sides with the tires and see if the pull also swaps direction.
Any pulling or drifting is not normal on most roads no matter who says otherwise. Constant drifting or pulling in one direction only promotes tire wear which eventually shows up on the edges of the tires in the direction of the pull. Toe amount will alter the amount of wear each tire exhibits in this case.
Too many alignment people simply set your front end to fall somewhere inside published spec guidlines and that's not nearly good enough. Specifically setting the alignment settings per tire diameter, vehicle design, etc. while staying within these guidelines will make one alignment technician shine above the others.
Camber is the lean of each tire sideways. Proper camber amounts counter the effects of the higher center of the road which allows water to run off the road during rains. Two laned roads typically have a higher crown than multilaned roads and your camber may have to be adjusted a little if you primarily run one or the other most of the time. The larger the tire the more aggressive the camber setting will have to be. Cold bending a straight front axle is necessary to correct camber on them and cannot be done repeatedly. It's often not fully effective for tires as large as 11R24.5. That's where cross caster can be a handy tool to completely eliminate all right or left hand drift and resulting tire wear.
Rear axle thrust angles, toe and camber cannot be readily adjusted on most rear wheel drive cars and pickups but many front wheel drive cars can be adjusted (many larger trucks, motorhomes and buses have adjustable thrust angle capabilities).
Toe and camber wear on the rear is similar to what you'll find on the front with similar problems. Thrust angles on the rear can cause dogtrotting which has the tendency to wear the steer tires as well as the drive tires.
Now that we have a better understanding of some of the angles I will try to explain how you know when you need an alignment. First find a smooth straight stretch of road like you travel most of the time and turn loose of the steering wheel. The vehicle should travel in a straight line for an extended period of time. If it does not it needs attention. If it repeatedly goes off to the right, camber is not set aggressively enough. If it goes off to the left, camber may be set too aggressive. In either case cross caster can be a factor.
Next rub your hand back and forth across the tread of all four tires. If you feel sharp edges rubbing one direction but not the other you need to have an alignment check. Visually inspect the tires for any signs of wear and act accordingly. If you purchased 50,000 mile tires but they wore out in 25,000 miles you need to have the alignment checked and be sure to watch the air pressure closer on the next set plus have them rotated and rebalanced several times in the tire's life.
There is a type of wear that is unavoidable on certain tires and vehicles (especially larger ones) called river channel wear. It happens to only radial tires. This wear will be equal in width on all tire edges if the alignment is in good shape. It may also appear in the interior tread in the form of narrow bands of wear at the edges of a continuous tread groove around the circumference of the tire. This wear is of no real consequence and should not significantly reduce tire life. The only time it becomes a problem is when a tire is out of round or balance, then it widens and begins a dip in the tread which eventually causes a shimmy or shake at the steering wheel.
If you hit a pothole or road debris and noticed a change in the handling of the vehicle you may need to have the alignment checked. Sometimes road debris can bend or warp suspension or steering parts and cause rapid tire wear. Be sure to tell the technician you encountered road debris so he will check all the settings on your vehicle including SAI/IA, steer axle setback, etc. A four wheel alignment check is not a bad idea even if the rears are non adjustable in such cases. A bent frame can be readily detected especially if there was a prior four wheel alignment check to compare this one to. Frames on smaller vehicles are not as strong as you think if impacted from the side.
Don't forget the wheels, road debris or a bad pothole can bend or crack a wheel and lead to even greater problems.
Always get and keep a copy of any printouts of your alignment settings where a computerized alignment machine is used. Learn what your particular vehicle's optimum alignment settings are (not published vehicle specs either, they are only a guideline). Good tires are much too expensive to leave to chance or a poorly done alignment.
Dipping in the tread and/or shake of the steering wheel is indicative of out of round tires and/or bent wheels, very poor balance, etc. Be sure to locate and correct (if applicable) the exact cause of the problem then replace the tires (tire replacement will be necessary where the tires have become out of round).
Believe it or not but things such as wheel bearing condition, shocks, brake drum or rotor condition, ride height, hub roundness, vehicle loading practices, etc. has a direct effect on tire performance. Slack or any other wear or damage to suspension, steering or frame will affect tire wear in a negative way and could be potentially dangerous.
Tire wear pattern diagnostics and all the aspects of wheel alignment and steering is impossible to fully explain in just an article but more information can be had at www.hastprotrucks.com especially if a larger truck or RV is involved. Cars and pickups can get a lot of relevant info as well.