It's not really me saying it's nonsense, it's the manufacturers.
The thread I linked to started off with me suggesting that the paper was not required because I assumed that the designer of the grain focuser would have built this in to the total height making it un-necessary (which is how I would design it). Ralph Lambrecht disagreed but claimed that the paper was not needed.
As no one could find any instructions stating if paper should or should not be used, I decided to ask some of the manufaturers.
Based on the replies we received, it was obvious that I was wrong about them designing in the thickness compensation but that Ralph was right (as far as the manufacturers were concerned) in that it was not necessary to use paper under them.
I don't doubt that you work very accurately considering the size of prints you are making but it would be interesting to see the difference if you focused both with and without a piece of paper printing just an 8x10 section of a 30x40 print.
I don't think that you will see a difference but real life results trump theory every time!