It matters more the more you lift, but even with stock suspension. The boot is clamped to the CV cup, which is the part that drives the CV joint. That cup is made of steel, so it gets real slippery when you get grease on it. When you go to put on the boot, it needs to be dry so that it will stay on long enough to get a clamp on there, and so that you can adjust the final postition of the boot on the axle. If you dont have lift, its not as important, as you are not fighting against extreme angles.
HTH
-J <img src="/forums/images/graemlins/kewl.gif" alt="" />