I think the key is being confident. If you confidently tell someone you have a stutter, it will make them comfortable because you're seemingly comfortable with it. If you don't mention it, or act embarrassed, it might give them some second-hand embarrassment or make them uncomfortable, just based off your energy. It's obviously really hard to be confident about your stutter, but I find that even if I am faking confidence about telling someone I stutter, it really changes the whole vibe of the conversation in my favor.