Anybody who comes up with the crap of doing it to put off their opponents(I can comment on Maria and Victoria) is talking absolute bull**** and has clearly never been to a tennis tournament in their life, as each time I've watched them practice in Paris, New York, Melbourne, Wimbledon, Madrid, Birmingham etc etc at each place from 1st ball of their practice session to last ball they have made exactly the same grunt/shriek/noise as they do on the match court and both have been doing it from an extremely young age. I know the Williams often start off quiet in a match and then the volume increases but Maria and Victoria start that way. Yes Victoria's grunt shouldn't really go on as long as it does but se's been doing it for years, that's now become a part of her on the court.
And to me the less you talk about it the less you actually become distracted by ANY player making a noise, that's certainly become the case with me with all the women and lot of the men who especially it seems on the clay seem to grunt more, I don't even pick up on the grunts now until it starts to be discussed again!