I can only speak from the other side, having conducted many interviews for new hires. Do what makes you comfortable, but I want to see an indicator of how you will perform. I would prefer a candidate that presents with as much confidence as possible. From my perspective, a “disclaimer,” would tell me your confidence may be lacking. I’ve never had the opportunity to interview someone who stutters, I have interviewed and hired people with both Tourette’s, and Asperger’s. Non-issues for me. Neither gave me a heads up. As far as staying calm. Imagine it is just a role playing game, a rehearsal. Just know the company, the job and yourself. Regardless of the position in question, at the interview you have one job. To sell. You.