WITH the ball season in full swing, we've been quite intrigued by the number of black-tied politicos we've been spotting doing a mean whirl on the dance floor. But, as Keeping Posted discovered, there will always be someone around to spoil their fun. Creeping up while we were watching a certain handsome Legislative Council member strutting his stuff, the ball-goer muttered: ''That must be the Bureaucratic Rhumba.'' We inquired somewhat confused: ''The Bureaucratic what . . . ?'' ''It's quite simple,'' he disclosed. ''You take one step forward, two steps back, and then sidestep the issue.'' Really!