I only aim for almost dead bosses or lvl 80 ones and try to deal as much damage as I can. Or also bosses that recieve a lot of damage in a short time/with a few people on it.
It sound a bit greedy but actually it's the best way I found to get currency and help other peoples to get them too. Trying to nuke bosses with all their life and less than an hour left is just a waste of raid point.
Obviously, allies bosses are my priority if they fit thoses conditions. I will also nuke almost escaped allies Queen Bee when I'm sure I can kill it (but with my whole team it's really easy).