Sure, let them swear. Just don't spam them. A few swears can add to moods or personalities, but swear spamming gets distracting.
I think what most people have said here are right. It depends on character and if they would speak that way. instead of rambling on I'll just quote Stephen King: "Talk, whether ugly or beautiful, is an index of character... In the end, the important question has nothing to do with whether the talk in your story is sacred or profane; the only question is how it rings on the page and in the ear. If you expect it to ring true, then must shut up and listen to others talk." Having said that I think there's a line to walk. I have no problem with swearing, I love words and swear words are no exception, just more tools in my lexicon but one of the reasons I like swear words is there expressive value. If you use them too much they loose their meaning. If you used the word 'chair' twenty times a paragraph then it would get pretty boring, the F word is no different. holding back a little bit can give these words a little more punch when you do decide to use them. Personally my formular is to use anything less than the F word freely and often (when character appropriate). The F word a reserve for when I want a bit more impact. And the C word I hold back unless I really want to slap the reader in the face. To finish with another Stephen King quote: "If you intend to write as truthfully as you can, your days as a member of polite societyare numbered."
Your characters should speak as they speak in real life. One of my MC is a Roman centurion. He drops hell, damn and son of a bitch on occasion. Another is a Roman senator, who swears hardly at all. it is not his voice. But when he calls someone fellator (cocksucker) it gets your attention. He is serious and means. It should be an appropriate part of your character's voice, and I agree, I don't overdo the f bomb. Though my female character uses the f word with another female character in discussing 1st century women's choice in men... they both chose theirs, though the other woman is having doubts. "Most women don't get the choice we have had with Antonius and Galosga (their two men). Most are like me with Wang Ming (she was his concubine, assigned at age 12 with no discussion, now dead at her hands). We fuck when they want to fuck, etc... ), that is the only use of the fuck in the book. The f word offends a lot of people when used to excess, so keep it relevant and minimal My editor also advised that I use whore in dialogue, where my character would use it, but not in narration, where the reader might interpret it as having the author's approval.
