Now that our daughter is 18 months old, we've got to watch what we say down to the letter, because she makes a go of repeating anything and everything she can. One of my more frequented F words has been replaced with "foxtrot," and we say a lot of "seriously?" around here. Of course, I don't necessarily consider expletives the worst words there ever were. In our house, words like "fat," "stupid," and "retarded" evoke a much more fiery reaction than words I can't write here for fear my grandmother might read them.
But what about our young adult readers? To be fair, if an author aims to create a world that their teen fans can relate to, dialog is king. Gossip, chatter, lies, secrets, confessions - these kinds of things are what drive a teen novel. And expletives are now so commonplace it's cooler to abbreviate them that to use the whole word. Still, it bears mentioning that potty mouthed young protagonists may put off older readers in our potential crossover markets. And there are some that think bad language takes the integrity of the writing down a notch - and yet there are others that would argue if it's true to cause, a character cussing a blue streak would infact validate the integrity of said book.
Is there a right or a wrong answer here? Honestly, I'm not sure. But I do think it's most important to stay true to your characters. I'm drafting the sequel in the Tanzy Hightower fantasy series, and there's a point at which one of my characters realized another has set her up for an epic fail, and everything in me wants her to scream: "b#tch!" (sorry, Nana.) But I haven't used language in either book thus far, and to drop a word bomb might detract from the scene as a whole - not because it's a bad word, but because it's out of character for the novel itself. But in a YA fiction book I'm working on, one character is particularly crass, and no words are off limits for her. So my opinion is this: to thine own self be true, and no matter what words you decide to use, make them count.