AAA rules say white on tail disqualifies an animal from registration. However, they are allowed to present birth marks, white hair grown on black hide, and white below the belly line behind the navel (as other have said). No idea if one could argue the white switch is a birth mark and get away with it, but I'm guessing not.
I'm guessing there are plenty registered that have a white switch, but the owners don't disclose it and paint/dye it for photos and shows.