I often tell students to stick their necks out, risk humiliation, and use their English as often as possible. Male students, however, often want to use those profane four-letter words, like 'fu_k!' and 'sh_t!' They seem to think it makes them 'sound' more 'native' or perhaps 'cool.' Some say they use these expressions even in their own languages. Great.
I haven't seen any statistics about non-native speakers who use profanity being better received or perceived by natives. But, at least, in my classes, I let students know that 'fu_k!' is not an appropriate word to yell out when their team doesn't get a word in Taboo or Hot Seat (vocabulary games). And it is totally inappropriate to yell 'Fu_k you!' at a female salesclerk who decides not to sell a student a package of cigarettes because she doesn't recognize a foreign passport as proof of age - no matter how absurd her decision might have appeared to this male student.
Another problem with non-native speakers getting in the habit of using some common expletives is that they might not be able to control their use in formal situations. For example, I had a cute blond female FCE (Cambridge First Certificate in English) student who used 'sh_t!' during her first practice test interview. In Part 2 of the interview, she was given a set of pictures that she found difficult to describe in English, and her initial reaction was to say, 'Oh, sh_t!' Although this student was tape-recorded and the entire class heard her inappropriately using the expression, she repeated the error during the second practice exam. ('Sh_t!') Unfortunately, this student did not pass the test. I hope it wasn't because she used that four-letter word when she wasn't supposed to.
Last week, a thoughtful, mild-mannered Korean student who teachers are very fond of and who has made great progress in learning English practiced a new expression from his host dad. What was the idiom? 'I don't give a sh_t!' Apparently, on the back of the host family's car, there was a bumper sticker with these words on it. When the student asked what the words meant, the dad explained that they mean, 'I don't care.'
Thinking that the expression was simply another way to say that, the student tried it out both in the classroom and in an essay. Of course, the outcome was quite embarrassing as his morning teacher was quite shocked to hear him using the expression in conversation practice in the classroom. The evening before, I was also startled to see the same expression appear in a description of his weekend in L.A. The student was trying to express the idea that although it was very hot in the city, he had enjoyed himself. 'Because I was with two beautiful girls and a guy friend, I didn't give a shit.' I could sort of understand what the student meant, but it was definitely unexpected in the context of the writing assignment.
Personally, I believe that ESL teachers should let students know what these four-letter words mean if the topic comes up in class but, in general, to advise students against using them in public or with strangers. This also includes other expressions, such as 'God damn it!', 'Holy shit!', 'asshole!' or 'bitch!' Know what they mean, but don't make a habit of using them. That's my rule.
Nevertheless, for those of you who disagree, here's a clip of a documentary film dedicated to the f-word, and some other expressions using the word 'shit' from an online dictionary.