I had always heard that AMA required either a spinner or a rounded prop nut, and that leaving the end of the shaft exposed was not acceptable. I looked at the AMA membership manual, but could not find specific words. The closest I could find was "Refrain from using a pointed spinner, propeller fasteners,...". In the interest of safety, in case the plane hits someone, you would be better off replacing the spinner, with either a new spinner or a rounded prop nut, before you fly. I'm pretty sure some clubs will not permit someone to fly without a spinner or rounded prop nut.