After Deadwood had been officially deemed a success the Los Angeles Times ran an article that addressed the "colorful" language used in the show, i.e. whether or not it truly was accurate and/or excessive. The "expert" they consulted said yes, it was accurate and no, it wasn't excessive. She explained Deadwood, the town, was the Las Vegas of it's day and that most people (men and women) went there to let loose and get away from the "rules" of the more "civilized" big cities; what happened in Deadwood stayed in Deadwood.
I've also read other experts (including the American editor of the Oxford Dictionary) who argue that much of the profanity in Deadwood is modern and unlikely to have been heard at the time. The F word, for instance was used sometimes but only for its original meaning, not as a multi-purpose exclamation. Much of the authentic swearing in the period more likely consisted of other very creative but milder epithets. I personally don't give a shit, it works brilliantly in the series.