Only just saw this post…
You know, one of the funniest, laugh-out-loud moments I had with a book was with Rachel Cusk’s The Country Life. Cusk - in her earlier novels (I’ve only read the first three) - styles herself very much in the Jane Austen tradition - long, eloquent sentences with a flamboyant use of words, usually twisted into a wry humour. She’s a brilliant writer. In The Country Life, the heroine is staying at a large country house. She is polite, well-brought up, everything is very mannered, and there is the typical hideous, stuck-up housekeeper who intimidates. At one point right in the middle of the book, the heroine runs into the housekeeper on the stairs or somewhere, and out of nowhere, she just says, “You c***.” It’s the only swear word in the whole novel, and because of it, and because no one was expecting it but it’s spot-on about the character, it’s hilarious.
At the other end of the spectrum, take Irvine Welsh. Go to amazon.com, search for Trainspotting, choose “Search Inside” and then take a look at the “Text Stats”. Then have a look-see at the 100 most frequently used words. Now that’s swearing. And that book was a runaway bestseller and turned into a cult movie starring Ewan McGregor in the role that made him.
Honestly, there is nothing that would annoy me more in a book than having some aggravated chav sitting in the housing benefits office, discovering that his benefits are being cut, and saying, “Oh bother! You really are rather unhelpful, aren’t you, old chap?”
Of course, there are sections of my own writing where I think, “Oh crap, what if my mom ever read that?” But Stephen King has good advice there: the first draft is written with the door shut - that is, without worrying about what other people will think. After you’ve written that first draft, then you can worry about the swearing… Uh, talking of which, I really should start on a first draft… 
Best,
Keith