it seems to be hit or miss if you "need" them. but installing shims to correct the pinion angle to the oem spec is not a bad thing. I've done 3 15-21, only 1 "needed" them, but installed shims on all 3.
I ran it for a few weeks with just the blocks removed and no shim with no issues. When I installed the drop shackles I also installed the beveled shim that came with them.