I was always told that for equitation sitting the canter is the proper way to show. However, in hunter under saddle classes, you're supposed to get up off of the horses back to allow for free movement.
For breed shows, I normally see people sitting the canter at all times. Although if you are showing in the normal hunters, it is proper to half-seat in the Hunter Undersaddle class, and sit during the Equitation on the Flat class. Good luck!