This was mentioned in another thread I was looking at the other day...and it was news to me also. Apprently, the deal is, if your side glass is sitting ON TOP of the bottom glass (thereby, the bottom glass is baring all the load), you want to use foam, or some similar underlay. BUT, if the bottom glass sits inside the side glass (as most tanks are done), then you don't want any foam underneath. This is because the side glass is now baring all the weight and pushing down around the edges of the foam, causing an upward or negative pressure against the bottom glass closer to the middle of the tank...
As I said, this came as news to me also, but the more I have thought on it, the more sense it makes. I would guess various factors would come into play, such as tank size (the effect would be alot less on a 2x1ft tank as opposed to say 8x3ft) and any extra bracing on the inside joints, but the theory is certainly a viable one.
Of course, if you know your stand to be slightly out of whack here and there, you'd probably just want to take the chance and use foam regardless, or maybe even skin the top with a thick guage ply to ensure a completely flat surface...
