@Miracle it depends on what classes, I think. Some things you only need to go to level 2 or 3 to get the good ability, but imo they should get to four at least once and it varies from class to class.
Like my fighter, Glory. I got them to level 4 for Bite and Kick and harmony and stuff, but recently, a few months later, I also got them up to Ranger lvl 4 (for Double Shot)
But then I was multiclassing my whole team, and I think I only got the others up to level 2 or 3, because their first job was what they did best. (I’ve also reverted glory back to fighter for the bonuses on melee attacks)
So you might go up to level 4 the first time, and switch back to that class when you’re not actively trying to get them up to a certain level, and then what level you’re going for depends on what class they’re training in.
Does that make sense?