The center speaker should go nearest the center line of the TV as possible - regardless of where you have to place the R/L fronts. The center handles almost all of the dialog and if its off to the side it will seem that ones guys lips are moving but somebody offstage is talking.
Most of the time you can adjust the distance and volume of the various speakers to the seating position from within the processor's menu. My Pioneer even has an automated way of doing this - way cool.