I would never put rocks on sand again, I had a tower collapse as the sand shifted under it. Probably induced by some livestock, but it was a mess. Didn't hit glass, but from now on my rock sits on solid footing and sand goes in second.

I stack mine very carefully, and a little bit of DD putty between them ensures nothing is going to move. I also place some putty anywhere a rock might touch the glass on the back to reduce the stress concentration.

Yes I done this before putting my sans and liverock and completely covered the botton of my aquarium with one sheet of it. It help spread the weight and also help anchor the liverock so if a fish dig underneat it will help to maintain the integrity of the structure. It would also help of some rock fall on the bottom to avoid shock and breaking the bottom of the tank.

when I had a DSB built boxes with egg-crate and zipties for the rock to sit on so the bottom 4" wasn't buried in sand.

With the new tank and BB, rock just sits directly on the glass.

Some interesting points have been made.

I was only planning on putting small(ish) sections of Egg Crate (EC) under the rock and not over the entire bottom. I'd like snails, my sea star, bulldozer shimp/Goby to be able to dig away without the impediment EC might cause.

The though was that where there was rock, the EC would displace stress and PREVENT any/all of the aforementioned critters from undermining it and causing issues.

The acrylic is a really good idea too as is the ABS -but I've been told not to put ABS in a tank but I'm sure a few bits under sand wont' hurt anything.