Well, guys, it sounds like a theory that can be tested. Get a jpeg file and leave it sitting on the desktop. Duplicate it and open and close the duplicate as many times as you can before you get bored... and then compare both files in PS blown up to 100%. Wouldn't that be reasonably scientific??
I hear ya, and I encourage the OP to do this if he needs concrete proof, but the only thing you need to know to be certain is a basic understanding of how computers work.
A file is stored on disk. All files are composed of a series of ones and zeros written on the disk.
When a file is opened, this series of ones and zeros are copied into the computer's RAM. While the copy of the file's data is in RAM, the program can do anything to this second copy of the file data. For instance if this is an image file, then the computer can ONLY do a brightness manipulation on the copy of the image file's data that was copied into RAM. The original data file is still sitting on the hard drive UNCHANGED. Only when you SAVE the file again is there a permanent change to the file that is stored on disk. In fact, that's why the command is *called* save!
To test this, all you need to do is look at a file's "date modified" time, then open the image file, do some alterations to it, then close the window and when prompted click "don't save". Now look at the date modified on the file, and you'll see that it says the same time as it was at the moment when you opened the file. That means the file has NOT been modified in any way!
And if you're under the impression that merely reading a saved file can degrade it, then think about what you're saying! Basically your entire computer is running on code that's stored in data files on your disk. For your computer to function at all, it REQUIRES that these huge amount of program code files are *always* EXACTLY the same at all times, forever. In fact, when something bad does happen to your disk which alters the saved files in an unexpected way, that's called disk corruption, and it causes your entire computer to become fubar in a very short period of time.
This is very basic computer knowledge. A storage drive keeps your files exactly as they are, and they do not changed at all with repeated openings and save-less closings of the file. The only way the file can be changed when it has not been resaved is if your physical disk is actually becoming physically, mechanically damaged. And of course, that happens very rarely and has absolutely nothing to do with what format a file is saved in.