Officially it's not allowed, Unofficially they wont really care too much for cosmetic stuff or that since it doesn't affect core mechanics of the game. Generally the majority of MMO's don't care for cosmetical modifications it's only when it's used for cheating or that where it become's an issue. My guess is your asking because of WOW's recent ban wave eh? Believe that only happened because the method used to modify the game was also being used for cheating as well, rumor's there is they quickly reversed this because they realised they'd just banned a 1/4 of their remaining playerbase as well as the fact they know the vast majority weren't cheaters.