Don't do that, Casters get first dibs on LB in dungeons regardless of team comp since their's does more damage than Ranged and is easier to aim at a pack of mobs, followed by Ranged, then Melee should only use it on the final boss
and LBs generally aren't DPS losses unless you do them in the middle of a burst phase (or if you're single target SMN/SAM and the animation lock completely fucks up your rotation and filler GCDs or whatever)
additionally, if you have any LB charged up and you know the next boss will reset it (mostly alliance raids) then as a Caster or Ranged you have no real excuse for not trying to pull with it
p.s Caster/Ranged LB1 does more damage than Melee LB3 at 6 targets