I would recommend using a center channel speaker to help with imaging. Since the path lengths are unequal to the ear the imaging will not be centered. The only time the use of a center channel (IMHO) is not needed is when using kick panels, since the placement of the speakers is more equal then having them in the doors.
Plus adding a center channel will pull the sound towards the windshield thereby creating a soundstage that appears to come from your dash. When you go to a concert do you sit on the stage or in front of it?