It seems to come down to a few criteria:

-Concern over pressure points on the glass
-Bottom breakage due to falling rock

If you're not particularly concerned with either of those, then there's no reason to put eggcrate at all !

I can't say I'm seriously concerned about pressure points and I'm not concerned at all about falling rock; it's just that in this tank, I've got more depth and can afford to pile more than a few rocks high and thought that perhaps the additional weight may pose a little more risk. I think that I'm not giving glass enough credit for it's strength....

After having the bottom crack on a tank I opted for egg crate. It just made me feel a little safer. I put 3-4 inches of course sandon top and never had any problems with sand blowing around.

I'm dis illusioned with sand tho. I had the deep sand bed go bad in my RSM a month ago and it was a mess tearing the whole tank down and rebuilding it.

I have some days off.. So I'm siphoning the sand out of the bigger tank a small section a night and replacing it with a a single layer of crushed coral. I find I can clean the bottom with the crushed coral by just moving it around with the suction hose. Granted I can't get all the sand under the rocks but I'm getting most of it.