Glasscages.com recommends 3/4" Styrofoam under their tanks. If you do not place it on styrofoam they will not uphold the warranty. I've heard of people cracking the bottom of their tanks by placing foam under it though. I think someone on here had that problem. I'd wait and see what some other people say.
I personally have never used anything under my tanks. I just make sure the stand is level and there are no gaps under the black trim before I fill it up. I've had 10, 20H, 30H, 30 long, and a 75g with nothing under them.