Well..as you know Jpeg is lossy format, which means even if you re-open it to take out a dust spot and save, the image will have to undergo uncompression followed by recompression. This may not damage the overall quality too much to notice anything, but it is still happening. If you try and change something a little more spread like creating more contrast for example, then you will loose much more data, especially from any shadow and highlight areas.
This is why I specifically defined as an added footnote up in post #37 what exactly I consider a drop in quality since I know everybody has different standards.
BTW, when I say "without losing quality" in reference to JPEG adjustments, what I'm actually talking about is quality differences that you can see in actual prints viewed at reasonable differences. Some people define a loss of quality as any difference they can make out when viewing something with their nose touching their screen at 100% (nose into a 3 foot wide print) or even 200% (nose into a 6 foot wide print). This is extremely stringent and
ridiculously stringent considering most people don't view photos like that. I usually review photos and adjustments at 50% or less. I only view at 100% if I'm checking focus for the most part. The most extreme example I've seen are those that use special photoshop layer/threshold masks to highlight the smallest of differences. These are
invisible period, even in prints, and even on the very best monitor viewed at ridiculously high magnification, which is why they have to use photoshop to "see" any difference. I no longer consider that photography but rather computer geekery or analysis paralysis.
I do most of my post-processing in DxO. DxO never touches the original file, and if I set it to output JPEGs that it processes at 97% quality, I end up with about the same filesize on output that I started off with on the input. Occasionally I need to run the Image Trends Hemi plug-in for fisheye photos, which I get to in Photoshop. I take the modified JPEG photo that was just saved at 97% in DxO, open it in photoshop, run the plug-in correction, save at Quality 10 to another new file, and that also seems to give about the same filesize on output as I gave it on input. With those two steps and with the type of editing that I do, I see "no noticeable loss of quality" even when viewed closely at 100% on my screen which is the far more stringent of tests. Normally I like to sit back a bit more and view at maybe 50% and look at the entire photo at once.
In the occurances when I have seen a noticeable drop in quality, it had nothing to do with JPEG compression itself and everything to do with bad or inappropriate settings in DxO. A good example of that is something like
this. I was shooting through a moving bus window so I needed to keep the shutter speed higher, took a loss of light from the window itself, a 2-stop loss from the circular polarizer I was using to neutralize reflections, and the end result was having to shoot at about iso800 on a D80 which is just about its upper limit before things really start to get nasty. The noise itself wasn't too bad, but DxO didn't agree. DxO was identifying all of the fine details on the cliff combined with the high ISO as "noise" when it was really real texture and detail and not noise, and the result on output was very smudgey and flat looking photos. You could see this clearly even at 50% at a normal distance, and it was plainly obvious at 100%. I had to go back and re-process this whole series and turn NR off completely in DxO and then they looked
much better. And since DxO never touches the original, it's no problem at all to do that.
Beyond that, there are many other quality limits that I'd be butting up against before ever being able to blame something on JPEG compression. There's the dirty bus window for one. There's the fact that the bus was moving and that in some cases there's just a tiny bit of softness in the photos from motion even if it's not plainly obvious. Then there's the fact that I was shooting with a respectable but non-professional consumer lens, the 18-135DX. It's nice, but just doesn't compare to the professional 17-55DX f/2.8 that I've got now which looks incredible at any focal length and aperture. The 18-135 is nice, but isn't as sharp wide-open, but I couldn't stop down due to conditions. If I did want to stop down I'd have to run iso1600 which would have a far worse impact on quality than JPEG compression ever would. So for all of the photos from that series, RAW just wouldn't have done much for me either due to too many other things in the way first having a far greater impact.