Jpegs are compressed files so when you open and close them they lose data and start to degraded through time and leaving noticeable artifacts. Using data files with greater bitrate structure such as PNG's will always insure that all file data is retained longer.
Um that makes no since whatsoever. If you open a file be it compressed or not, your computer copy's the file to RAM, uncompresses it and deletes it from RAM when your done. Absolutely no data is changed on your hard drive in the process. If what you are saying was true then it would be impossible for you to read a JPEG from a read-only medium such as a CD or DVD.
Unless you were talking about opening it in say photo shop and re-saving it.