I've heard diferent opinions regarding this.

One is if it's a flat bottom tank IE Acrylic or Eurobraced tank then you can use foam, but if it's a tank like an oceanic that has a trim then you should not. I even heard that if you call oceanic and ask them that they will tell you not to use it since it will void your warranty. On my 10 foot long 3 foot tall tank I debated this question and called Tenecor, the manufacturer and they said it doesn't really matter and since the tank was pretty heavy already and going to a very tall stand I was not going to mess with trying to raise up any higher since it was only my girlfriend and I moving the 900 pound tank from one stand to another, therefore I did not use any foam.

I've never used foam in any of my tanks and i've had many ranging from 10 gallon to 750 gallons and never had an issue. With that said glasscages tanks (eurobraced) are required by the manufacturer to have foam underneath.