I think there's general agreement here that it depends on (a) the genre and (b) the character -- is it in context for that character?
In my novel, much of the swearing is in Cantonese, by Chinese, (followed by translation), who use it more than English speakers. Attend a football match in Hong Kong and a missed goal is greeted by 30,000 loud "F***s". Thus a cop shot in the arm exclaims, 'Diu lei lo mo!" (F*** your mother), which is as common as 'damn.' in English.
Despite what I thought was quite a lot of swearing, applying Harry's Fuckety Index gave my novel a score of only 0.23, so well within the acceptable range.
Very interesting too about the different cultural attitudes. A Spanish friend tells me swearing in Spanish is just like an exclamation in English. Cono is very popular in Spain and she says is the equivalent of bother!