Usually in Dungeons, I go like this:
Use LB on first boss because it's usually still about 75% full by the time we get there and fills up while we're fighting it. Or if it's an earlier dungeon, it doesn't actually fill up at all during the first boss because our ilvls are too high.

1. Use on first boss.
2. Skip use on second boss in order to build up the second bar for use on the last boss at about 5-10%. At least that's how I do it.