I won't LB if the boss still has a decent amount of health and I know some sort of mechanic change is coming that can mess it up or if the party isn't doing well and I think we may need the Healer one to survive. If neither is the case, than LBing at anytime is fine because you're not going to do anymore damage at 5% than you are at 20% and it's stupid to waste the damage. Plus, it lets you build up gauge again faster for another LB, even if it is lower level.