I think it's useful, pedagogically, to keep students on their toes. But mixing registers is also so much of what I do in my daily life, whether in talking about current events or in discussing my scholarship with friends (you should hear me describing a day's work to George Washington Boyfriend--"So he's doing more of that crazy shit in this other poem, where he's all, 'check ME out. . .'"), and I think it's good for students to see that. I think it's good to model how to use the expletive as occasional intensifier, and to show the comedy that can result when you throw a casual vulgarity into a complicated sentence full of arch circumlocutions and technical terms.
I hope to model, I guess, how to be smart and yet "normal"--how my students could incorporate scholarship and criticism into their own lives. I want them to feel that there's not a huge yawning gulf between their experience and their abilities, on the one hand, and the texts we read. (There may in fact be such a gulf, but they need to be able to ignore it.)
So, although I have no expectation that I'll break the "fuck" barrier in any of my classes--not unless I'm reading aloud from a text--here are some of the things I've said in class in the last week or so (third-person pronouns always refer to a character, author, or narrator--not to anyone in the classroom):
He really is a little shit, isn't he?---------------------------
She's got some balls.
So, is he just a total bastard, or what?
It's all about proving who's got the bigger dick
I think he wants to have his ass kicked.
Why is he acting like such an asshole?
UPDATED TO ADD: Check out Horace's post on a similar subject.