Showing your teeth can mean one of two things;
1. aggression and power
2. submissiveness and safety
The first happens with clenched jaws, as stated above. It shows the strength behind the muscles of your face.
The second, however, happens with loosened muscles and perhaps even a slightly open jaw. This says “I have these weapons, but I’m not going to use them on you, you’re okay”. A smile.
Think of the difference. If a handsome stranger clenched his jaws at you you’d think he was a scary freak, but if he smiled you’d want to get to know him.