Its all about preference and space avaliable. You obviously don't want to put the speaker 5cm away from the back of your seats as this can cause the back seats to rattle as I well know. I think you should try the sub pointing in one direction and then the other and see which one you prefer. A
As you mention there is not much in it but the acoustics are different for each car and it doesn't hurt to give it a try.
Dave