There usually isn't. But there is a hierarchy to it. For instance, if she promised a dance to someone else, then priority goes to that other person. If someone else also asks at around the same time, she has free choice to choose who her dance partner would be and can effectively deny everyone else. Not postpone, deny. If she is already spoken for, someone's fiancé/wife/lover, she can also deny without being rude. However it's typically seen as "not becoming" if that dance request is from someone of higher status despite everything I've already wrote. A woman of higher status of the guy asking, is completely free to deny his dance request.