I used composite shims to level both my 250DD's as well as my QT/Frag rack system. Wood shims may deteriorate over time, especially in a garage on a concrete floor.
I do not have any scientific basis for this, but I would feel uncomfortable using more than one shim to level a stand. If you need two or more shims to achieve a level condition, then you may want to consider some other options. Just IMO.