Choosing whether or not to have characters swear is a tough call. My book, An Unquiet Grave, falls mainly in the cozy category and many cozy readers are put off by swearing. On the other hand, as an author I want to create believable characters, and if one or more of those characters are law enforcement officers, wouldn’t it be more realistic to have these officers swear, at least occasionally, under times of extreme provocation? Because most cops swear. Same with EMT’s, fire fighters, doctors and nurses and other high stress professions. It’s a way to release pressure.
So the question remains, if you have police officers in what is an otherwise cozy book, should they swear? Mine do.
But not often, not excessively and only for effect. In a 72,000 word book, I think my characters use a total of eight swear words. I feel that, for my smaller town officers, having a few strategically placed swear words provides more impact than a continuous stream of expletives – but again, this is for my characters in my particular books. I think it makes them more believable if they occasionally let loose a frustrated four letter word.
By the same token, characters in more hard-boiled or noir books would not be as believable if they didn’t swear; frequently, colorfully and in original and inventive combinations. I think it’s a judgment call every author has to make, based on book type that leads to character believability.