There is a tall tower, a medium tower, and a short tower, right? All of them give the car a different roll center. Lowering the tower lowers the roll center, raising it raises it. The opposite thing happens in the inside camber link stud, adding shims lowers the roll center, taking them away raises it.

Raising the roll center makes the chassis roll in the corner a little easier, lowering it keeps the chassis flatter. You can read up on it a little more here:

http://en.wikipedia.org/wiki/Roll_center

But to answer your original question, the factory guys always run the shortest tower, and if they don't, they cite it on the setup sheets. The only time I ever saw a taller one cited was on a 4wd.