OK... I've tried sitting on my hands about this thread, but it's just not working!
While I think the original poster's question has been beat to death, it sure seems like there's some misunderstandings as to what foam will and won't do. I apologize in advance for the length of this, but hang with me here for a minute!
Maybe it's easier to think of the foam as a bunch of little springs under your tank, because really... that's all foam is - a different type of spring. And like any spring, foam has a spring constant which tells you how many pounds it takes to compress it an inch. For a given spring rate, the more the spring (foam) is compressed, the more load is transferred through the foam.
Now lets think about our tank that is sitting on a perfectly level stand, evenly compressing the foam all the way around the frame. The compression is even, so the force transmitted from the tank through the foam is the same all the way around. That's perfect!
Now over the years, the stand sags for whatever reason, and you notice that the gap is wider between the stand and the tank frame in some spots versus others. In other words, the foam has rebounded and is now compressed less in some areas than others. Remember... the load transferred through the foam is directly proportional to the amount of compression.
So, the area of the frame where the foam is now compressed less does NOT have the same load going through it compared to other areas under the frame that have compressed more. The foam does NOT even out the load - it transmits a load that's proportional to the amount of compression. The bigger the gap (less compression), the less the load going down to the stand in that area. And if the load is less in that area, then it's MORE in another area.
To add another wrinkle, most foams have some amount of permanent compression set - over time, it will NOT rebound to 100% of its original thickness. This compression set varies between foam types, and even foam manufacturers. So what this means is that even if you measured your foam before you put it under your tank, and years later as things start sagging and the foam "expands" and you think it still has some compression left in it... it might not. In reality, that foam under the sag might not be carrying any load at all. And that puts you right back in the situation that you were trying to avoid - having no support under the frame of the tank.
My long-winded point it that having foam UNDER OF TANK FRAME ONLY really doesn't hurt anything, but it also doesn't guarantee one bit that your load distribution is equal along the length of the frame. Depending on the amount of gap, and the type/strength of foam you use, it could be the exact same thing as having nothing under the frame. Foam can not transmit constant force and compress less, at the same time - it's just beyond the current laws of physics!
And if you've read this far... thanks!