Unless the Boss has a sliver of HP left which can be DPS-LB3 downed then it's ok to use healer LB3. It's often applauded as well. However, if both healers are still alive with at least 1 tank I would give the healers a chance to raise the group. Really depends on the fight you're in. Some fights you're just running around dodging stuff without the tanks taking a beating it's not hard to get everyone back us with raises either.

Finally, keep in mind there is a range on the Raise-ga (i say 80-90% of the boss area) so it's possible you might not get everyone if someone died on the opposite side of the boss arena.