My take on mastering is that it doesn't do much at the lower levels. Obviously the larger the number, the more x% of that number will be. I kept all my buffs at level 10 until just before changing to 3rd job because it would've been barely noticeable and would've cut down my potion usage. I don't really believe it to be worthwhile to master a buff unless you were a lion, as AC is hard to come by.
Attacking skills are more worth while to master (as long as you're using them, I waited until level 135 to master my cat's volley kick), just be careful not to master something like quad punch, as it's just a prerequisite for tetra and would be a waste of points to master. Upper smash is the same way.