It has nothing to do with tempering; it has to do with the design of the tank. Some tanks are designed with a floating base; All-Glass is one of the largest makers of this design. The bottom piece of glass is slightly raised from the bottom, attached & supported by the sides. Since the edges support the side, no support is needed in the middle. All-Glass makes tanks with and without a tempered bottom using the same design.
Tanks that use the bottom piece of glass for support do need foam to even out the surface the tank rests on, and to prevent any small piece of debris from keeping the base from contacting the stand evenly, creating a pressure point that could lead to the bottom failing.