Personally when the player being tutored is very like their tutor, in terms of position, attributes or just general style of play then I'll set it as on field. This should help them to learn the position better or whatever it does. If the players aren't very alike and it's just a matter of matching up any partners for the sake of it I'd set it as off field.
I always have my older players tutoring my younger players, never really sure of it's outcome or if it's effective but it must be doing something right?