Well, that's up to you. Fantasy/sci-fi/dystopian novels generally have no cursing. It seems in scary, futuristic societies, people watch their language. And middle grade novels don't have any curse words because those readers are too young. Cursing seems to be an issue in contemporary YA. We want our books to feel real, and let's face it: real kids curse. But does that mean we need to use curse words in our books?
The more serious your novel is, the more likely it is you will involve cursing. If you've written an issue-driven book about drugs, abuse, or violence, then chances are you will use dirty language to emphasize the scariness of a character, or the harsh reality of this world. But for regular, contemporary YA, I don't think cursing is needed unless it has a point. To this day, controversy surrounds teaching Catcher in the Rye in schools because Holden curses. Yet his cursing is viewed as a symbol of rebellion against the phony, upper-class society he inhabits.
Personally, I rarely curse, so I don't feel comfortable writing characters who drop f-bombs left and right. And when I read a YA book or watch a teen movie, I'm taken out of the story when the characters begin swearing excessively. Filmmakers like Quentin Tarantino and David Mamet LOVE swear words, but it's natural to their adult characters: mobsters, hitmen, gangsters, shady businessmen, angry power-crazed men. Some YA characters will curse more than others since it's part of their characters, like if they were lower-class or poor. But for a suburban teen, is it necessary?
To garner a PG-13 rating, a movie can only contain one f-word; any more, and it's instantly rated R. PG-rated films can only have maybe 1-3 s-words. G-rated films can't have anything filthy. When PG-13 movies cash in their f-bomb, they usually have a strong reason, i.e. the protagonist may be in a climactic fight. In She's All That, Laney Boggs shouts the infamous "Am I a bet? Am I a f--king bet?!" The f-word gives that line more power, makes it more jarring. For the most part, curse words are used sparingly, and that doesn't make audiences enjoy these movies any less. Luckily, books don't have that kind of censorship, but I think there should still be some responsibility on the author and editor's part not to take advantage and load up on swearing. Yes, kids swear, but that doesn't mean they won't enjoy a book with clean language. You don't need f--k and s--t to have your characters sound like real kids; you need them to have conflicting emotions and complexity.
I'll leave you with what my dad always said about swearing: The English language has hundreds of thousands of words. Why do you have to continually use the same 4 or 5?
What are your thoughts on cursing in YA? Do you have it in your story?