I have used both styrofoam and cork as an underlayment for tanks with a solid base. If your tank has a floating base, you really don't need it. In other words, if the tank's perimeter brace is what's going to makes contact with the stand, an underlayment is really not needed. However, if it'll make you sleep better, go for it! By the way, the cork is a whole lot easier to cut than the foam, which is a big plus.
Both can be found at your local home improvement store.