This got me thinking about how many strange things people - people who should know better - tell us glass artists. I don't think they deliberately mislead us, I think they are passing on to us what they heard, and didn't question. Kinda like the average internet myth. My first instructor told me that to apply patina, I should leave my piece soaking in a bucket of patina over night. (Correct use: wipe it on with a cotton ball, let it sit for 10-20 minutes, and then wash it off.) My neighbor's teacher forgot to tell her that when planning her window insert, she should include a margin for the lead border. When her insert didn't fit, she was told that there was no way to repair it without completely dismantling it. She took it apart, and lost heart in her project. 2 years later, she still has no window insert. (Correct info: even after you have soldered it together, you can remove the lead border on one side, carefully measure and cut the piece, and reapply the lead border.)
So, in respose to the question about copper foil, I don't throw out old copper foil. I even save all the little bits left over from rolls, sometimes using them years later. If there are spots that have green fuzzy stuff growing on them, I throw that part out. But if it's just tarnished, the flux should clean it up as you prepare your work. To give your copper foil a longer life (and prevent those darling green fuzzys) keep it in a plastic bag like a ziplock. And don't handle your foil when your hands are wet or might have chemicals, like flux on them.
Do you have a question? I might be able to help - email me!