as a general rule, i'd've thought the fewer codecs/encodings the file had been thru the better... if a lossy codec has thrown data out, a better codec can't just reinvent it. once compression has clipped the file it stays clipped.
this is why in pro photography we tend to hang on to a raw (if available) or TIFF as well as a jpeg - the jpg file's great for web, and possibly printing realllly small (depends on file size obv!) but useless for quality print. think how many times recently you saw fuzzy product pics in the news columns of whatever mags u read - this is what happens when someone in marketing sends the mag a 19k jpg...
