The layout of your live rock structure is everything. It determines the overall look of the tank, where the pumps are to be pointed, where the lights will hang, and so on. Before even filling your aquarium with water, I would highly recommend that you build a live rock (or dried rock in many cases) structure outside of the tank, on the ground. This will let you see how things go together and allows you to change things extremely easily. And once you get an idea of where you want thing to go, maybe take a few pictures to remind yourself of the layout as you put it into the aquarium. Or, you could always cement the rock together using epoxy, plastic rods, or zip ties.