You want to experiment with locations that give the best overall flow in all areas of the tank. Water flow is primarily meant to keep ditritus from building up around and under live rock. Move them around every few days and find locations that do the best at eliminating stagnant areas on/in the LR and across the sandbed. When I was using powerheads, I had one at each end (in the back) pointed towards each other, and another in the middle pointed at the front of the tank, which caused more random flow. Pointing them towards the front might work well too because that will cause a downwash around & under the rock.