I have some insulation foam under my acrylic tank. My original plan was to have a glass tank, so that is why it's there. I changed my mind at the last minute and went with acrlyic, so the foam just stayed put. The reasoning behind the foam (about 1/2") is to protect a glass tank from any minor twist or bump in the stand that might put a pressure point on the glass, causing it to crack. I have a friend locally whose glass tank cracked when it was getting filled up for the first time (no foam under it). The tank company replaced the tank, and the replaced tank was put on styrofoam for added protection. So far, so good. I honostly can't think of any reason to use it on an acrylic tank, though, unless you want to use it for good measure. I also can't think of any cons to using it either.
Hope this helps