Like everyone said, personal preference.
For better or worse, the marine aquarium hobby puts relatively little emphasis on aquascaping. Contrast that to planted FW aquariums, where aquascaping is truly an art with a whole library of well defined styles, rules, guidelines, and page after page of forum threads dedicated to the subject. I would encourage any reefers who are interested to go read on some FW sites to catch on to some of the fundamentals that are harder to learn in the marine aquarium circles.
All that said, one guideline which I think is often overlooked is to try to envision the aquascape once it's covered in healthy coral growth. When you're setting up a new tank, it's easy to establish the right visual balance by filling it up with rock - but, then, once the tank grows in, things are out of balance.
In a mature system, the corals themselves can take up at half or more of the visual space. If you're truly going to account for that, the aquascape should probably contain half as much rock as you think it needs, visually speaking. At the very least, the amount of rock you use should feel uncomfortably small.