tank lb is only good for huge spikes of damage that will wipe your party otherwise (garuda nm, tam hard if you dont kill enough adds)
heal lb is only good if the party is getting hit way too often (as in, they arent dodging enough)

melee lb is the default desired lb, the only time otherwise, is if you need to kill multiple things fast, instead of just a boss

depending on some content, you can lb twice
for example, you can melee lb at the start of the fight on the first boss in brayflox and have no issue having the single bar be full again by the time you reach the last boss, at which time, youll be able to fill a second bar, and should use melee lb2 once its up