There is no specific "correct" placement because everyone's rockwork is different and will deflect the flow in unique ways. I've got my powerheads facing slightly up for a few reasons:
1.) I want good surface agitation for oxygenation.
2.) My tank is extremely narrow and the Koralias are big. If I aimed the PH's directly at my rocks, it would blast the tissue off corals.
3.) I want a slightly lower flow at the bottom of the tank for future LPS corals and clams. I want the upper zone chaotic enough to keep SPS.
However, I still need enough agitation at the bottom to keep junk from settling. My DSB has formed itself into dunes rather than a flat surface, but that's alright with me. I prefer a high flow because it keeps my sand cleaner. I've got another small PH periodically hitting the back of the rocks because I don't plan to grow anything there and I don't want trapped food settling.
In a square environment, there are always going to be areas that get more detritus than others. Try to keep these where you can see them and remedy it. I gently "poof" my sand surface with a turkey baster in one particular area quite often because the overflow channel tends to trap stuff there. The system is telling me not to place a species that needs high flow there and I may not be able to do much about it.
I've positioned my powerheads many times since I started this tank and I'm sure I'll mess with them plenty more as corals grow out and begin to block the flow.
The point is that you need to think, fiddle, and experiment to get the best flow for the health of your system. I agree with demonsp that you need more flow, but don't panic about stuff. Test and tweak
