I mean, there's no hard rule for these things. If it makes you feel better doing it in advance, you could do it. Or if you stutter during, you could explain it. Or you could not, and if they ask about it specifically, you could. Or if nothing happens, you could do nothing at all. I'm not sure I'd present it as an "illness", though. It's just... stutter. Right? People know some people stutter.