[Van down by the river]

Hello Jeff,
I've built many many tanks and I'd say use Styrofoam. It helps distribute the weight evenly across the bottom pane of glass. It also compensates for uneven spots or pressure points on the stand from wood knots, poor construction, or settling/shifting.
There's a reason Oceanic only guarantees Oceanic tanks on their own stands. I often see that people's aquarium stands seem to be an afterthought rather than a serious part of the design process. The stand should already be on a stable, even floor to start with. If large amounts of shimming is required I'd suggest another location as the floor may continue to settle. Shimming should be done with a level prior to the tank being filled and checked/adjusted after being filled. Remember glass is intolerant of the stand flexing. Same as moving tanks can be quite stressful on the seams. Styrofoam is cheap and there is no harm in adding it, so why not.

Just a note; If they are shimming the actual tank and not the "feet/base" of the stand then IMO they are asking for problems.