The thing with the shims in front is that it messes up your steering geomety. A couple of degrees in the shims doesn't seem like it would make much of a difference, but it does. See when your axle is unshimmed the steering can go a true side to side like it is supposed to and it pivots on the bottom of the tire. But when its shimmed, the pinion is rotated up along with the rest of the axle. So when you turn your wheels they aren't pivoting on the bottom of the tire, its actually up higher on the tire, causeing more wear. A couple of degrees in the shims isn't going to make a whole lot of difference, but when you start getting anything above 2-3 degrees, thats when you start running into those problems.