I started putting foam under our tanks when we got into reef tanks. The reason I started was that on both reef tanks we have, I refinished the stands. Because of this, it's difficult to say for sure that the top of the stands are completely flat and level. The tank will settle into the high density foam (I use the blue kind) and prevent the stand from causing any unneccessary pressure points on the tank. I don't know if I'd have problems without the foam, but using it makes me feel a little better.
I didn't us foam under my 210, but as you can see from the picture of my Perfecto stand I don't think the foam would have done much good in my case. My stand only supports the tank on the ends and in the middle. I considered putting a piece of plywood down under the tank and using foam on top of that, but the support still would have been in the same places, so I decided not to.Has anyone not used foam on a 125 or bigger?

fsn77: where did you get the "blue foam". I ended up with insulation foam and it never totally compressed.