When I do military presses I feel them primarily in the front and side delts, so for balance I do behind the necks so I can feel my rear and side delts. Dumbell presses are like military presses in that they involve a lot of front delt, so you never get to feel the rear delts during pressing movements. I would never give a client with shoulder problems behind the neck presses, but if you personally have no problems and understand proper form go ahead and do them. The form issue is that most people do them with their elbows angled too far back which causes impingement, when the right way to do them is with the elbows down. Just be careful with these.