Therapists are pretty understanding, although they won't know anything about stuttering. I wouldn't worry about. I also think disclosing is a good idea. The only thing I would be a little wary about is if your therapist tries to diagnosis and treat you via the stutter. Also as someone new to therapy you should know, the skill range of therapists varies greatly. It's actually kind of hard to find a good therapist. So if after a session or few you're not feeling it, you don't have to keep going back. You can try another one that might be a better fit.