am181d
Adventurer
Regardless of what feels "realistic," the fact of the matter is, PF is a game first and foremost. You have to be fair to all the players, and that means that if you don't require training to level up in even one class, then you shouldn't require training to level up in any class at all. Conversely, if you do require training to level up in one class, then you should always require training to level up in all of them.
Well, I'd amend that to say "PF is a game. It's meant to be fun for the players. Take what the players will find enjoyable into consideration when making a ruling." I imagine that some players will enjoy the role-playing aspects of arranging for training.
That said, in my current PF game, my human rogue street rat has been threatening to pick up some basic training in sorcery, cleric magic, or paladinhood (!) to his fellow adventurers every chance he gets, because "it doesn't look that hard." If I *did* carry through with taking a level of any of their classes, I don't think I'd argue if the DM wanted to build in some in story logic.