If you use it before the second boss you are almost guaranteed to get a full bar back regardless. Saving it just for the last boss is a bit like not using one of your cooldowns at all during the battle just to make sure it's ready for the beginning of the final boss fight(as far as logic goes I mean). You really should just use it if it's available before that point unless you are really worried about the party being anything like the OP's case. There's no harm in doing it whatsoever and it's a nice excuse to use it more often than what you usually see, especially for Ranged and Casters who usually only have the excuse available to them if there are no melee in the party in the first place.