Just to reinforce what the others have said, I had two cels
hanging next to each other, one with UV-proof glass and one
with regular glass. They were in an interior hallway, with
no windows, no direct sunlight, and no flourescent lights,
just the indirect sun from the other rooms. After about one
year, the cel under regular glass shows definite signs of
fading, while the other cel still looks great. So based on
that, I would definitely recommend the UV-resistant glass.

Admittedly, it's hard to picture exactly what the cels 
looked like a year ago, I don't have before and after scans.
I know the faded one wasn't in perfect condition when I got
it, but I'm pretty sure it's gotten worse since then. And
just seeing how well the other cel has held up is enough to
convince me that UV-proof glass is worth the price.

Skorp