I've been contemplating the same thing w/ my fuge. My thinking is with no sand in the fuge that's one less place for detritus and nutrients to accumulate. I have about 20lbs of rock so the LR
pieces/rubble should provide enough "nooks and crannies" for micro-fauna and keep the macro's in place.
I think the amount of rock you can fit in the fuge should dictate whether or not to use a sand bed for more surface area. Hope that helps.