Today I was doing some leveling on WHM and out of curiosity checked LB damages. I did 2 runs of Shisui of the Violet Tides. On first run, none of the DDs used LB before last boss, when melee used it (LB2) for 84 800 damage (189 DPS for the fight). On second run, a caster used it on last big pull before second boss for 141 131 damage (around 400 DPS for that pull I think) and there was a full LB1 bar at the start of last boss fight (we got LB2 which the melee didn't use...)

So yeah, caster LB1 seems definitely worth using on big pulls and is the most beneficial way of using LB in dungeons, I would assume using melee LB2 didn't cut 189 dps from the melee (who had 1100 dps excluding the LB) in final boss fight, so that's probably worth using as well if you get one, and using LB1 at least right before second boss still lets you have a full LB1 bar before last boss, so there's no reason to save LB for last boss even if you want to use LB2 on that.