By habit I place mine on top. But I would agree that if one could find a middle ground between aesthetics and function, on top of a grid or support structure would be most beneficial because one could clean in between the crannies most efficiently. I find this to be of vital importance for long-term success of the nano aquarium, but not to say that rocks have to be placed like this in order to succeed. Proportionate to the size of a nano, the ability to keep one clean and free of detritus seems to be the key in battling all sorts of ailments from algae outbreaks to sandbed "crashing" (imbalance of nutrient mineralization)
I do both, so I didn't vote. When I put it on top, I try to place very small rocks underneath where the larger rocks are touching, so that they "float" above the sand. My pistol shrimp is doing a good job of excavating underneath my rocks right now...
