It should not matter much if I understand your concern DiegoFrogs. I think you are saying that the measurement could be inaccurate if the rollers have a difference in the amount of play/wear, meaning the rollers have worn unevenly. From Sheldons explanation, what is being measures is not physical stretching of the plates but mainly the amount of wear at the pivot points. The difference in wear of one roller compared to another should be statistically small. If in doubt, you would probably be better off changing the chain. Letting it go could mean a more extensive drivetrain overhaul and replacement.
In pre-index days I used to tighten up my shifting by removing some of the deflection from the chains. I would do this by pressing the sideplates slightly tighter. I did do it a little to much on occasion but it was readily obvious when I had problems getting to the higher deflection gears. I don't have time to mess with that anymore, plus index shifting is much better than I need at my aged level
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