Personally I think each instance is different. In Brayflox HM for example: most times we kill the 1st boss before LB 1 is up. If its up before the kill I'll use, but it usually ends up being the kill shot(or very close to it). If we have the LB at 2nd boss, but I have a BLM or SMN I'll ask before the fight if they want me to save LB for for Big pull since we'll get another in final boss. At final boss I always LB but not until I burn my CD's, jumps, so I can insure it kills before the bomb phase. In most cases I usually just LB at 25~30% to avoid what this very thread is about. I will admit there are some times where I'll be vain (only on bosses that are a joke like demon wall post 2.2)and save it for the kill shot because c'mon... who doesn't feel cool after LB'ing something to death. In cases where I'm new to the instance I'll ask experienced players if I should LB at some point specifically but most times they'll just say "hey DRG LB this fkr at x%" and I'm fine with that.