Only wasted LB is if you do it in an invulnerable phase of the boss or like in Ravy in shieldphase where you have that difference of doing 1-2% in shield to 8-10% in swordphase...
Well and if the mobs were dead anyway

Soooo many melee refuse to use LB anyway (My stacks, My BOTD, My Huton ^^). And in a HM shouldn't matter at all and even in all curret content everything about LB use is optional... many ways lead to a kill

You did nothing wrong OP you did all right by using the LB at all (plus the adds can make you lose the island)