Some boss fights have "stages" in which the enemy is more dangerous, and for that reason must be brought down faster (the one at Dzmael comes to my mind, but also Haukke Manor). I've had a healer accusing me to LB too soon while in Brayflox's Longstop (we were at the 2nd boss), and I pointed out that none of the battles in that dungeon had relevant moments in which a DPS's limit break would be more appropriate than others, and that I'd use LB at the beginning so that the rest of the fight would build up some LB pool for the next boss. So, yes, it changes from dungeon to dungeon. Unlike what some people think, DPSing is not a mindless role, and even this kind of decisions must be taken with some reasoning and strategy.