Excellent posts by everyone who's replied; I would just reinforce the bit about providing for good current in all areas of your tank. Especially, keeping rock off the tank walls will help to keep the dreaded dead zones to a minimum. I've always placed my rock in the sand, though. Next time I aquascape I'll try putting the really big pieces on the tank floor.

Oh yeah, one other thing I read once advocated keeping some areas of rock separated by open sand to prevent migration of corals (like xenia!) throughout the tank. If you have a big enough tank to do this, it's worth a try.

Matt