LOL. I'm kinda surprised people still think this is about being nice to people in dungeons. In four man stuff it seems to me most of the time people follow the convention of dd give it to dd and the tank gives it to the healer and vice versa. In stuff like FL the healers give them to each other.
Personally I just click at random half the time, apart from frontlines where its lala healer> other healer> someone at random if the healers left before I could click.