I have has a sneezing attack during a peformance, but I kept on playing to my own and people suprise. I don't think I could deal with an ichy nose I would have to scratch it even if it meant taking one hand away from the keyboard, quickly scratch it and get back to it. You will stuff up if you havent practice taking ur hand away from the keyboard, I guess its all left to your concert prepatation. But it would be murder feeling that itch throughout the piece, it totally distracts, I'd probably rush the piece so I could quickly scratch myself eheh.