The first Scripture that comes to mind is Colossians 3:8 which says, "But now you yourselves are to put off all these: anger, wrath, malice, blasphemy, filthy language out of your mouth." Since this is a clear injunction from Scripture, Christians are obligated to obey it. The question then becomes, what is filthy language? Since it would be nearly impossible to put a biblical definition on an issue that depends on subjective judgments; I think that culture, conscience, and common sense come into play here. Given what the common culture accepts as taboo language, I would find it very difficult to justify a Christian using the "F" word for any reason.
Although I can't imagine Jesus or the apostles using words like the "F" word in every day language, I wouldn't put it past Jesus nor the Apostles to use crass language to prove a point. Case in point, Thomas Cahill, a respected New Testament scholar has an interesting translation of Mark 7:18-19 in his book, "Desire of the Everlasting Hills." Cahill quotes Jesus as saying, "What don't you get? Don't you see that nothing that enters a man from the outside can make him unclean, since it doesn't go into his heart but into his bowels, and then passes out into the shithole?"
In the sidenote, Cahill says,
"Usually translated "privy" or "sewer" the word that Matthew chooses is aphedron, Macedonian slang that would have sounded barbarous to Greek ears. Jesus was not bashful about referring to bodily functions, even if His translators are."
I think there is a deeper issue that is far more serious than the question of crass language verses standard language. There is a big difference between saying I fell on my ass the other day and calling somebody an asshole. One is improper etiquette. The other is a form of contempt, something that Jesus strictly warned against. I think the most important Scripture dealing with language is what Jesus said in the Sermon on the Mount.
"And whoever says to his brother, 'Raca!' shall be in danger of the council. But whoever says, 'You fool!' shall be in danger of hellfire."
I think the Christian who occasionally substitutes "ass" for "rear" and "shit" for "dung" has less to fear than a Christian who avoids taboo language, but has a habit of cutting other people down by calling them "stupid" "idiot" or "moron." Contempt is a far more serious issue than bad manners. In my judgment, I think it's best to avoid both. What's important to avoid in this discussion is hypocrisy. I think Jesus would agree.