lw2jgog: 10% isn't /that/ huge, but it's nice. I gather that there are a couple of training dummies in Act III, but that's a bit late.
Off-hand, Cover, Assassin, Strong Stomach, and Haggle also have very limited windows of opportunity, so if you're restarting, be sure to nail those as well.
There are more than enough in ACT III, almost 6. And the +10% adds up to every XP gained till achievement, so you won't miss anything, except you will gain a bit later those +2-3 levels.
Personally I have capped the 10 dummies in Act III too, and I got +2 levels instantly.