well i have no choice in my car, they have to face up because of a custom box in my hatchback. but i like them to face out towards the front. so sounds waves travel forward...
in a hatchback/suv u have it sub up port back, in a car if ur seats fold down have sub/port pointing in, if ur seats dont fold down then sub/port back.
^^^ thats just in general... however feel free to test new things like sub forward port to drivers side and stuff like that. the only way to find out what really works in ur car/suv is to test.