Limit all the things! Limit everything! EEEVERYYTHIIIING!!! MUHAHAHA!


On a serious note, what Anova said.
It differs from dungeon to dungeon tho. Sometimes you want to LB the 2nd boss if that ones more annoying, even if your Limit break won't get full for the next boss.
Sometimes you also don't limit the boss directly, but an add or a "nail" or "meteor thingy", since if you don't get that down fast, your party might wipe.
And sometimes your 2nd or 3rd limit bar might not get full before the last boss dies, so you can limit earlier too.
Generally, you LB once the bar is full at the last boss. Sometimes you even manage to get another bar full for an additional 1-bar LB.
If you're not sure what to limit or when, you can always ask. If no one replies or they say: "no special mechanics", just LB the boss

You also can't buff a Limit Break, so using Blood for Blood and stuff don't matter. It's also calculated by every weapon from every party member. So the itemlevel of the person using the Limit Break doesn't matter. Meaning, you can also use Limit break even if there is a lvl 60 Dragoon/Monk/Ninja with better equipment than you.
Oh and be careful, sometimes a boss get's into a phase where he's invulnerable, if you limit it then, it get's wasted (LB 3 needs long to cast too... so watch out! )