If it has a plastic trim, the plywood does nothing for the tank, but may help the stand structurally. You should NEVER put foam under a tank that has plastic trim. It can compress enough to actually push up on the bottom glass and cause the seal to fail.
If it is a rimless - glass or acrylic, then the solid (plywood) base is needed for support, and a thin (1/4") layer of foam is helpful to even out any slight imperfections in the surface of the plywood to avoid pressure points that can crack the bottom of the tank. If you think you need more than 1/4" of foam, perhaps you should try a little harder to get the surface flat.