Funnily enough i covered the first lady ever yesterday, but that was becasue she was white blonde.....the lightest hair you can imagine, and she asked me to as her hair is really porous. I did it not because i felt I would get her hair, but because it clearly made her more comfortable
Otherwise, i've never had a problem at all.....I never get any solution in the hairline, and considering how easy it washes out of sheets, clothes etc, presume it would just wash out of most hair, if the tiniest bit ever strayed.
All that said, I mostly use an airbrush, so have very fine control, but never had issues when i have used my spraygun.