Start with bottom pieces about a fist's distance from the back glass. the ones stacked on top of that one can lean on the back glass IMO
. No rock should be totally against the glass so that there is no water flow. Come to think of it, I can't believe there are rocks that are flat enuf to do that anyway. Besides that area nearer the back glass in the rock will be where most of your fish sleep or rest - where there isn't much flow.
I prefer stacking against the back glass, but not against the sides. Being able to clean the side glass is important to me.