Every character who is remotely badass has stubble, even the girls.
It is physically impossible for Bob to shave his stubble, or for his stubble to grow any longer. Or it grows back the instant he shaves it off.
Bob can (and does) use his face as sandpaper.
Generally wimpy characters automatically become Bad Ass after not shaving for a few days.
Zig Zagged: ???
Stubble is purely a fashion statement and has nothing to do with badassitutde.
The cast is entirely female, or only the female characters are badass.
Enforced: The audience needs to know right away which character is a gritty badass, and stubble = gritty.
"A true badass never shaves! Razors are for the weak!"
"He's so manly, you could sand wood with his face!"
Invoked: "Bob, you'd look a lot tougher if you grew some stubble."
Exploited: Bob has lost confidence in his abilities for whatever reason. So he lets himself grow a bit of Perma Stubble, which makes him look Bad Ass enough to restore his self-confidence.
Defied: Bob is a total Bad Ass, but doesn't like the feel of stubble so he continues shaving.
Discussed: "That guy must be badass. He looks like he hasn't shaved in a week."
Conversed: "That character must be badass. He looks like he hasn't shaved in a week."
Deconstructed: Everyone who grows stubble is immediately Badass and society is thrown into turmoil. For instance, teen males who grow stubble earlier end up dominating their schools, be they nice guys or jerks.