Michael

When you focus the image you are focusing on the grain of the negative which is at a different plane than the glass the neg is sitting on.
I do not think that optical verses regular glass would make much difference... but I have never tested this and would be surprised to find it made a difference.
If it makes a difference then optical glass would be decisive consideration for the bottom piece.

I use AN glass for the top and this I am sure is not in play.