The real problem I guess is determining whether or not the car you are holding is an important collectible, whether by rarity or condition, in which case you should preserve it for future generations. For these types of cars, you never really own them, you are a "caretaker", keeping them preserved for the future.
Having read all of the advice on this thread, I think that perhaps the best general rule is that if a car is a very common car, thousands made, and it is only in condition "good" (which, in the collecting world means pretty darned bad, a grade of "D" in high school), then it is proper to repaint it. It may even be that by doing this, you are increasing its value and preserving it.
Just my thoughts.