Its up to you, I mastered every one of them, evern the special dress spheres. You evantually get bracelets that allow you to access what you've mastered in other Dress Spheres without having to equip the nodes with them.

It depends how far you want to get into the game, you don;t have to but it is useful. You can just get them to learn a specific ability that you need and never touch it again.

You get 1 AP point almost everytime you hit a monster with a character so I recommend you get in a fight with a high HP fiend and go crazy. I like the round gel things in Macalania Wood, just attack them and you do little damage but you get lots of hits in.