Grunting is actually discussed in the USTA Rules of Tennis and Friend At Court publications. Grunting can be considered a form of hindrance.

Item 36. Grunting. A player should avoid grunting and making other loud noises. Grunting and other loud noises may bother not only opponents but also players on adjacent courts. In an extreme case, an opponent or a player on an adjacent court may seek the assistance of the Referee or a Roving Umpire. The Referee or official may treat grunting and the making of loud noises as a hindrance. Depending upon the circumstance, this could result in a let or loss of point.

Personally I feel a good grunt once in awhile is in order and necessary to get that last bit of whatever out. So for all you players who like to grunt, I say UUUGGGH!