Depends on the fight and the bar you'll have. Say if you have 2 or 3 bars of LB for that boss, you want to save it if you run into it with a full LB bar cause more bars means more damage/healing/mitigation. If you only have 1 bar max for that boss (think Demon Wall in AK), better front load it so the fight is still simple and you can continue building it back up for next boss. It all depends on party, you don't have to rely on LB for most bosses (save Ifrit HM's LB) but it certainly is faster with it. I don't think LB can crit but haven't tested it.
