I would disclose it during the interview. Typically first interview is a phone interview. I always say something along the lines of “before we get started, I did want to let you know that I have a speech impediment. If anything is hard to understand or unclear, just let me know and I’d be happy to repeat myself.” This typically makes the interviewer a lot more understanding and makes me a lot more comfortable (and hopefully stutter less) because I’ve addressed the “elephant in the room”.