Quite often you'll find cars with just a 2mm spacer on the top of the front shocks only. This is done to help keep the shock vertical. The front shocks tend to lean backwards slightly without these spacers present.
That's the only time you'd ever shim the shocks different at the top and bottom - rest of the time you'd match the same number of spacers on the top and bottom of the shocks - unless you alter the wheelbase of course
