Correct, usually, the rear is not so grossly out of line that it causes issues, plus if the variations are minor, most drivers won't know they even have an issue. However I'm sure we have all seen examples that are waaay out of wack too.

Rear alignment can impact the way it drives nad handles by the classic "dog leg" or crab walk. This can cause excessive wear on the tires as well as making the car feel inconsistent between left to right turns.

During a turn, a leaf spring car's wheelbase changes as the springs compress and extend. If you have excessive angles with the rear housing, this motion becomes creates the inconsistent feeling between left and right turns since this motion is not transmitted equally side to side.

Certainly bent housings can impact things as well if it is bent into a toe or camber direction that again, creates a disparity between left and right. However, this can sometimes actually create an advantage to the handling if the bend produces a nominal amount of toe out or negative camber. I have known oval track racers who purposly bent rear housings to add camber to a live axle. It accellerates wear on parts as well, so its a touchy proposition on a street car.

So yes, understanding where the particular angles are on the rear is helpful and knowing them means you can correct/modified to some degree. If a shop will offer a 4 wheel print out, I certainly would take them up on it, but like Steve pointed out, for a live axle RWD car, it isn't going to include any adjustments on the rear, just information on where it is.