Why it is beneficial or harmful to rotate my arm at top, if i dont rotate will it impinge any muscle or nerve?
Watch at YT - for instance this, or simply buy his book. It is really inexpensive. There are some videos of Kelly Starrett - I've putted overhead press - since at the top mechanism, and a specially potential problems - looks the same.
There are some points here - you need to stretch your body bottom up. Be sure not to use your spine instead of shoulders to keep hands up. If you have pain in your back, at the end of ribs, perhaps that is the problem. Having proper general body position, move blade-bones back, and down. That way you open shoulders, with proper stability. There is something named rotation cuff, all that is attached to top of arm bone, plus biceps - having it open means less chance of conflict, so less chance of injury. If you are unsure - do tests - they are easy. Please watch this video. Well, all that names, acronyms are not so easy, but they do great job describing what is the problem.