Aha! I'm glad you mentioned Dresden Files because when I wrote my above comment, I'd been trying to remember some fandom I'd been writing for lately where the characters canonically insult each other, and that's it -- Thomas and Harry specifically were what I'm thinking of; I noticed when I was writing them and trying to get the voices that they call each other "dumbass" and "dolt" and so forth on a regular basis.

But yeah, it does depend on the character, and there are some who wouldn't, and some lines that even a normally rather sarcastic character wouldn't cross. No matter how insulting my sister and I get with each other, for example, I wouldn't ever tease her about her weight, because that's something that she's sensitive about. It would just be nasty.