This is a neat article on dialog in historical novels--do you use terms that modern readers wouldn't understand because they're authentic? It's a balancing act. (Of course, you can go the other way. In my books, the Special Forces soldiers all have fairly rude nicknames, and one of them is called Ofay, which was a turn-of-the-century derogatory term for a white person (in the 1920s it was shortened to "fays")).
And at the end the authors notes that people in the past didn't curse the same way we do. It's true! I realize that "fuck" has been around forever, but if you read, say, Chaucer complaining about how much people swear, what he talks about is the horrible blaspheming--swearing on the wounds or various body parts of Christ. That's what people did.
And that was the problem I had with the cussing in Deadwood--they even called people "motherfuckers," which is an expression that did not come into common use until after World War II. I mentioned that to my dad, and he noted that when he was growing up, the older generation was remarkably fond of farm-related curse words--"jackass," "bullshit," "horseshit" (while bullshit implies nonsense, horseshit is more malicious--if you've paid for something and the store owner isn't delivering it like he's supposed to, that's horseshit, not bullshit), "chickenshit" (cowardice).