It just depends on knowing your role and assessing the situation. The general consensus, though, is to wait for party lead to announce when to use it. As someone pointed out earlier, don't just say 'lb' cuz then you're going to confuse peeps who may not fully be aware of their role; for example, a BLM using their lb on a single target is going to pale in comparison to a DRG or MNK; whereas, if you need to destroy multiple targets (i.e. Ifrit's Nails) then it's almost mandatory for ranged DPS to do it.