The ratings go from lvl1 (worst) to lvl9 (best), and the increase in ability gets smaller as the number increases. That is, a lvl2 is much better than a lvl1, a lvl6 is a bit better than a lvl5, and a lvl9 is a very tiny bit better than a lvl8.
You need to decide how important it is for your staff member to be that extra bit better. I can tell you that most people generally will not need a lvl9 staff member, because it's a waste of money. Unless, of course, your star player is injured, and you really, REALLY need a 1% better chance he'll be back for the playoffs...