The main thing you have to remember with jpeg is that every time you work on a file and save it, if you don't save a version, you'll loose quality. Let's say you start working on a particular shot, decide to call it a night and save it to it's original name, you actually are compressing the already compressed jpeg.
This is why I feel NEF or RAW is safer for me as I always save a psd photoshop version, and then I'll save another version in jpeg. So, if I ever want to work on that file again, I can start fresh with an uncompressed file.
Jpeg files can deteriorate quite fast just saving and re-opening and saving if you don't change the name.
Marcel