well. i say if you know the boss battle is easy. and the LB is already full. may as well use it. so you can do damage and let it build back up for the next boss fight. unless the next boss is right around the corner. like in CT.

but there is actually good reasons to save the LB till towards the end of a boss fight. most bosses go through phases. of course the already mentioned "skip a phase" strategy. and fights can get tougher the longer it lasts. so essentially it makes sense to save you highest damage output abilities or buffs for last. they have lengthy cooldowns.