It also really depends on the size of the tank (dimensionally). If you are going to put 50lbs of LR
in a 29g, then you pretty much have to put it against the back wall or you won't have any floor space left.
If, however, you are putting 200lbs in a 4' long 120g, then you are going to have more options to work with it and not put it solely against the back.
Also, don't forget about how you are going to view the tank. If you can only see one side, then the rock may look best at the back. If it's viewable on more than one side, say 3 or 4, then you might want to build a mountain in the middle.
In the end, it's mostly preference for how it looks. HTH