yah, if the box is a truck sealed enclosure then its designed to face the seats but it would sound better if you face the wall so the sound can "bounce" but if you have a truck sealed enclosure then its designed to face the seat for a reason...maybe to save space?
you have to remember that most truck boxes are sealed and are made to face the seat also if you play it against the back of the cab the subs will be playing on a hard surface where if you play them into the seat there will not be as much resistance against the surround of the speaker
I remember a fad in the uk at one point where spl competitors would remove all the seat upholstery as it absorbed the sound, also firing the sub at a hard surface from a close distance can reinforce the bass response. Just a thought.