I'm assuming, marineland is the stand maker?
One reason not to shim some stands, is that it goes from distributing the weight through the whole stand, to transferring all of the weight to the pressure points where the stand touches the ground and where the shims are.
It's not obvious when it's so close to the floor, but it's analogous to placing a tank directly on a concrete pad VS placing it across a few 2nd story floor joists. While a short height, it's still enough for the materials of the stand to deflect, just like floor joists would.
Ask them how it would affect the warranty if you were to have a professional cabinet maker profile the bottom of the stand so that it would sit level on your floor without shims.