Honestly, I'll always save it for the healer and melee lb3 at >10%, unless it's progression and we're perfect on mechanics but really scraping enrage.
I've seen the most amazing groups in all sorts of content, perfect smooth runs that you'd never dream could wipe, suddenly fall to pieces on a mechanic midway and that healer lb3 saves the wipe. If I'd popped the lb3 we'd be starting over. In most cases the early melee lb3 doesn't add much unless your group really cut enrage close and even a healer lb3 for 2 dps can be a gain over it due to no weakness.
