Using LB as soon as it's available is valid if you are absolutely certain your party will not mess up anywhere afterwards. There are times (albeit rare) where a tank or healer LB3 can recover the fight. For fights that have phases, you may want to LB to skip whichever phase is likely to give the most trouble.

There is no such thing is always LB when available or always LB as a finisher. It's situational and will sometimes differ from fight to fight and from party to party.