Use it when appropriate- if there are a lot of mobs to hit, you'll not be remiss using it. People will always give you crap about using an LB when they want to, some people are just like that.

I had a guy get mad at me for wasting an LB on the golem boss in Qarn today, as a Machinist. While he was also a Machinist. He proceeded to piss and moan and call me a noob the whole dungeon and refuse to explain why it's a waste if no one else was going to LB and his would be the exact same as mine. Like, I went out of my way to be polite and ask what he planned to use it on and I just got an almost cliche "ugh noobs" out of him.

Point is, some people just suck. You know when to LB and it's not always "Save it for the boss, melee". Just be smart about it.