In dungeons, the most effective use of the LB is to AoE a trash group. For this reason, a melee DD paired with a caster/ranger should avoid using it immediately on the first boss. Consider that an AoE LB1 (just level 1) does more damage than a melee LB3 if there are 7 targets, or more damage than a melee LB2 if there are 4 targets, and you definitely don't get a melee LB3 option in a dungeon anyway, and it is uncommon to built up to LB2 while the last boss still has enough HP that the second bar would even make a difference...and the clear optimum is to AoE a trash pull. Then if it happens to fill up again before the last boss (even if it's immediately before the last boss) to AoE LB the trash again. Then if you manage to build another bar to hit the last boss with, melee LB the last boss.

That said, in any case, you're only shaving off maybe 15-45s from the run. So while the above is best, it's not worth arguing over.