It is a bit of personal preference guided by what your intentions for the tank are.
If you're going to have some burrowing bottom-dwellers, they'll need some space . . . if you're going to get some larger fish, they'll need swimming room . . . smaller fish like caves and crevices . . . corals will need a place to sit . . . and so forth.
Like FBM, it took me a while to get the final arrangement. Putting rock in is like dealing with a 3-dimensional jigsaw puzzle so don't expect to just spend a few minutes to put it in and be happy. Took me several hours and several stints of pulling some (or nearly all) pieces back out and working with different configurations.
Since we're going to have some few swimmers, we went with a two-mound arrangement with a gap between them. We incorporated a cave in case somebody really wants some alone-time or feels threatened, and we have a fair amount of open sand area for the eventual blennie or goby.