Hi Michael.
Having made a great many enlargements with a great variety of enlargers; I think the glass used in a carrier does not need to be better than "window glass". It is an amazing product made with very high quality standards. At one time plate glass was ground flat. With the advent of "float glass" which is continuously cast on a bath of molten tin; ground plate has probably disappeared completely except for scientific purposes. When I bought glass sheet, I would get what is called select quality which was examined for mechanical defects only. I also had an enlarger with a negative carrier made from two optical flats 10" x 10" (1985 price almost $3000 each. Ask me how I know.). Apart from scratches or dust, there was never a quality issue with any carrier I had.