I have to get something called "select" glass. It is regular window glass except it has been inspected, and should NOT have any of the normal window flaws. Just a bit more expensive than regular glass, but drastically less than "optically clear". It is worth it not to have a miniscule glass bubble in the center of things.

But I have to agree that surface scratches do not tend to affect the outcome of images. Dust and dirty glass are much more of a concern.