Use it sooner rather than later. There's no point in saving it exclusively for the last boss in a 4man dungeon. You can use it on every boss if pulling normally. Nothing grinds my gears more than a melee "looking cool" and using LB when boss has 2% HP left.

There are exceptions where you want good timing for it in later fights, but for early dungeons please tell your DPS to not "save it".