Nevada makes it simple -
NRS 486.191 Driverís position while operating.
1. A person driving a motorcycle or moped shall ride only upon the permanent and regular seat attached thereto.
2. A person shall not drive a motorcycle or moped with the seat for the driver so positioned that the driver, when sitting astride the seat with the motorcycle or moped in a stopped and upright position, cannot reach the ground with both feet simultaneously.
However, talking to my friends in the Reno Blue Knights, they say that while you can run into a LEO that is so anal retentive they try to strictly enforce the wording the intent of that statute is to state you can't ride sitting on a fender, gastank, handlebars or a naked frame. They don't see how standing violates the intent of the law.