I usually use it on trash if the tank pulls especially big. Since I play casters, I don't usually use it on bosses unless there's no melee or they forgot and the boss is almost dead.

I don't really understand the mindset of "My deeps" or "I hate not having two bars for the last boss" though. The whole point is to do a bunch of damage and speed things along. At least of the dps limit break. Better to get two LB uses and help two fights/pulls go faster than save it do the whole friggin dungeon and save it on one boss to me.