The only job interview I had where I told them beforehand was when my stammer was quite severe. It qualified as a disability which got me a guaranteed interview. I already worked there and would have had an interview anyway, most likely. My supervisor knew anyway and had let me have time off for useless speech therapy. I got the job but it took me years before I actually answered a phone (I was low level and it wasn't part of my job anyway). By the time I left that job my stammer was a lot milder, and I don't bother mentioning it now. It becomes apparent when it does and no-one's ever judged me for it (at least not to my face). I'd only mention it if it was relevent.