Glass making has changed considerably in the last 100 years. When I was young, window glass had quite a few bubbles and images "rippled" due to thickness variations, so I see the logic in what Denise says - for glass made 100 years ago.
Virtually any good glass today is superior to anything made 100 years ago, and probably even 50 years ago. I used ordinary window glass cut at my hardware store and it worked just fine. The Formulary now supplies 4x5 and 8x10 glass plates for coating.
I should add that Mark Osterman of GEH uses window glass, also with great success.