Depending on the situation I do like to mention it beforehand, like in a job interview. I feel it puts me in control of the situation, and I feel a lot more comfortable when I start stuttering, because it's not a surprise. And I also think most people I've told beforehand like that, or at least they like you for being open about it, it shows guts. Don't be afraid you won't get the same opportunities, I think you'll find respect instead.