If you're a caster watch out for large pulls when the bar is full and use it then. Melee LB is a strong single target attack, caster is the weakest on a single target. I don't use it single target really, it's hardly a dps gain with as long as the animation is.

If you make it to the last boss of a dungeon and no one has used it you can also pull the boss with it, like cast the lb on the boss as the tank runs in. Sometimes this is enough time to generate another bar.

Basically don't worry about it. It almost never matters. Once you get into endgame things there are some fights and strategies that require specific lbs at specific times but for the most part if you use it cool if not no biggie. Can be great on a large trash pull or add phase in a dungeon though.