If the whole party is both geared and competent, then you will use the LB to make things go faster.

If you are playing with PUGs or don't have the content mastered yet, then you will save the LB for the healers so they can save the day, if need be. If the fight goes well, typically a damage dealer will use it to make the last phase of a boss go faster.

As a Bard, the only time you should use the LB is when you have 3 bars up and two or mor party members are dead (especially if that includes one healer) - but try to get into a central position so you raise all of the dead guys. Using a one or two bar limit is usually considered a waste since that only heals those that are alive, which is the Healers' job anyway.