I think what you are trying to ask in a roundabout way is how do you get the hardwood flooring runs to lie in place so that nobody can sense there are any out of parallel walls. if this is the goal, the wider your boards, the less you will see non parallel walls. after that its a little bit of an art form. however, try this:
1) assuming you are using prefinished flooring, running the long joints down the long axis of the room, and that the room is rectangular. measure and mark the center of the room on the long axis. chalk a line from these two points.
2) take measurements to the long walls along their lengths to see how far the walls deflect over their runs. usually one wall is out somewhere. you should see a taper over the room, but you might get lucky and have perfect parallelism. if one wall is tapered, you will have to decide which one to use as the start wall. the outside wall is usually the one with all the furniture pile up against it, so it can hide much of the visibility of a tapered floorboard/baseboard interface, so you could start with the other wall.
3) measure your rooms width and divide by the actual board width. if you get a perfect number, then you are good to go. if not, that means the last board run will need to be ripped. you will have to take careful measurements and figure out approximately by how much before you lay any wood. you don't want to end up with a 1" wide board as the last board run, because if you have to taper it, your eye will notice the taper much more than if you have a 4" board as the last width. you are best to split the difference, so that you have to rip the first course and the last course (easier for fitting under doorframes, etc)
4) when you start your course of hardwood, leave more of a gap at one end of the first run to compensate for the taper. but don't do to much. the human eye can pick up approximately 1/2" in 8 ft in my experience.
5) lay your floor. its now going to be the best in can be for a given space that has non parallel and/or undulating walls