Placement is a matter of personal preference. The key though is to make sure it is very stable. There are several types of critters that can knock over unstable rockwork (hermits and burrowing gobies are just 2 examples). Make sure your bottom layer is very stable in the substrate. I partially burry my bottom layer of rocks just to be sure. Then, when stacking rocks, make sure you have at least 3 good points of contact with the rocks below and the weight of the rock is mostly centered over those 3 points of contact.
FWIW... in my 75g tank, I have the rock randomly scattered in the tank, none touching the glass - that's just the way I prefer it in my FOWLR. In my 44g reef, I have it piled up against the sides and the back. It's the only way I could get a decent amount of rock in a pentagon tank.