Every time I head to the theater, everyone—myself included—has a distinct preference of where to sit. So where's the best seat in a theater? When it comes to choosing a seat that is most optimal for audio and visuals, we trust THX. They recommend "a 36 degree viewing angle from the farthest seat in the auditorium." This is where the sound technician sits during the calibration process. It's probably also good to sit slightly off center to make sure you enjoy the stereo effect.
As far as visual guidelines go, here's what the Society of Motion Picture and Television Engineers (SMPTE) has to say, as described by Ralph Davis, AMC's Vice-President of Facilities, :
The horizontal subtended angles created by a customer's lines of sight to the left and right edges of the screen should not be less than 30 degrees. The viewer's vertical line of sight should not exceed 35 degrees from the horizontal to the top of the projected images. Ideally, the sightline should be 15 degrees below the horizontal centerline of the image.
Of course, personal preference always takes first dibs. But it's always nice to back it up with what the experts have to say about it.