I have my rocks in the middle of my 180, and I've noticed an added benefit with my fish. I have a few shy species and they do very well since they can hide in the back when they're frightened and venture to the front when they are feeling brave. Even with the choice, my fish are almost always in the front of my tank (especially around feeding time!)
In my 36, I have a similar setup as sasscuba, with a powerhead blowing across the back behind the rocks. The back of that tank is spotless...the cleanest area of the tank!
I think if your tank is small enough that a powerhead could keep the back of the rocks clean, then that works just great. But if you have a larger tank, it may not be feasible or it may require multiple powerheads behind the rocks.
It probably just comes down to personal preference...people keep their aquascaping both ways and they work just fine.