I don't do this - if I replace the tube I throw the old one out. When I was a kid, my dad always used to do this. Didn't work so well. When one of us would get a flat, half the time he would pull out the tube and the patch on the old tube would be broken somehow. Not sure why - maybe he wasn't patching the tube right. Or maybe the patch, when stored bend over inside a seat bag, doesn't hold up (or the glue doesn't hold up). I've had better luck with patches on a tube that's actually being used in the tire - being smashed up against the tire seems to do a better job at not breaking the patch somehow.

Since the advent of flat resistant tires, I get a flat maybe once a year at most, not worth the worry about my backup tube for me. (Though to be fair - I've had new tubes fail as well due to manufacturing defects, so go figure.)