This depends entirely on how your tank is built. If the side panels are built on top of the bottom, or around the bottom such that the bottom panel is in contact with your stand then you do want foam to cushion irregularities in your stand and relieve point pressures (this does not substitute for a level stand). However, if you tank is built with a floating bottom you DO NOT need foam under it and I actually think its better not to use it.
High density extruded polystyrene insulation foam works well but I've also used high density EVA foam interlocking mats and I prefer those.