Which seat should you use in canter?

You can either sit down in the canter or rise out of the saddle into a two-point seat (a light seat where the whole seat comes up out of the saddle) or a three-point seat (a light seat where you still keep a little contact through the seat. The usual way to describe it is that your breeches stay lightly in touch with the saddle, but your underwear does not). Which seat you use depends on what you are doing during the session and on what feels best to you.