I wouldn't worry too much about it---when I've accidentally left the red window open on my folders, I've never had a problem. I think backing paper is plenty opaque enough unless you're subjecting the window to very bright light.

But if in doubt, it certainly can't hurt to tape a "shutter" over the window. I'd use a piece of cardboard under the electrical tape, just to avoid getting sticky goo on the window itself.

-NT