I usually add dry hops when I'm about 6-10 points away from my estimated terminal gravity. Just stick 'em right in the primary. I'm sure there's a little loss of aroma because the beer's still gassing off a little, but guess that just means I need to add more.
A good clue for me is when the krausen falls, but I can still see sediment moving around inside the carboy. I'll add the dry hops, and then begin my final temperature ramp to finish out fermentation.
When I do it this method, it usually means that I'm dry hopping at 70-74* for a few more days. Once fermentation and cleanup is complete, I'll bring it back to 65-70 for the rest of the dry hopping until it's time to cold crash.