You are right, in general all you have to do is keep the stretch in the same position on all the pieces. My choice is to have the vinyl stretch side to side on car seats. The reason being, it's a lot easier to cut something down if it stretches a little too much than to do it over because the vinyl won't stretch. On anything else, like a door panel, it doesn't much matter.
No one lives forever, the trick is creating something that will.