Whenever you feel comfortable to disclosure. I usually state that I stutter after I actually stutter/have a block. I usually can say it in a throw-away fashion and continue talking (the sentence now acts as a buffer word for me). I have no obligation to make someone else feel ok about my speech nor explain it to others. Their reaction and judgment is their own. While it will hurt to be viewed negatively, there is nothing wrong with having a a stutter/stammer and they suck as a person if they think less of you because of it. I have also had people refuse to believe I stutter because they didn't remember me stuttering after knowing me for several months.