Another South Park reference! (There are three shows I constantly reference and in order to have a decent conversation with me you must watch the following: Family Guy, South Park and 30 Rock)
OK! Swearing! In my own life I swear like a sailor. However in my fiction nary a character drops the F*bomb. In fact, they don't swear a lot at all. Obviously I don't mind swearing. I've read books where every other word was fuck. Layer Cake is linguistic genius, full of British slang and tons of swearing. I swear a lot on my Live Journal http://www.spiderling.livejournal.com but it is near absent in my fiction.
There is very little in Tea Times Three and I carefully consider my choices when characters do swear and what words they use. So far not much cursing has happened in T3 but there are has been a bit.
My friend Sunny http://sunnyfrazier.com/ has some very firm opinions on swearing in books. That very often a lot of gratuitous swearing will alienate certain readers. I'd like to add that I think a lot of swearing becomes back ground noise. Which is fine if you are writing Layer Cake which is all about drug dealing Londoners. But ask yourself this: does the swearing have a purpose? Does it add to the characters? Reflect their socio-economic back ground? Show a culture or sub-culture they take part in? Or is it just bad habit (as is my case...) :) which could make a nice character trait especially, if like me again, they are trying to curb their potty mouth.
In a weird reversal of this I was recently reading an urban fantasy where all the characters used 'fantasy' swears throughout the book peppered with the usual mid-level swears of 'shit' and 'damn' . So when I came to the one and only 'fuck' in the entire book it was glaring and felt very inappropriate.