Depends on the job I'm playing, but generally speaking I find I have to wait until the perfect spot in my rotation to use a limit break unless it's going to kill the boss. For example, if I'm a monk, and I try to limit break, if it doesn't kill the boss, the length of the animation will often times mean losing all stacks of greased lightning, which means it's something I generally won't do, but as a ninja or samurai, my rotation is more forgiving, and so I will be more likely to use a limit break.

The only exception here is if I'm with a bard or machinist, I will gladly use the limit break even if it means dropping stacks as they tend to get trigger happy and pull the limit every time its up, even though they have the lowest potency limit break of all the DPS.