How big a tank, how big of rocks, how sure are you that they WILL NOT fall, even when accidentally bumped when you try to net that problem fish out?
On my 3' 58, I don't have anything on the bottom. Then again, there's only so large/heavy pieces I have - and I trust my aquascape to be quite, quite stable.
When I move to a 120, if I run that BB I'm going with starboard, no question.