This article is a little out of my normal zone, but it came up recently in a discussion and got me wanting to check it out. In short, I wanted to take another look at what level of compression would be best for my "normal" shooting.
I'd recalled a previous article about Adobe's compression and changes that happen at various points: Petapixel, click here, which talks about chroma sub-sampling changes between quality 6 and 7. You can find the original article referenced here, it's a bit technical but a good read if you're interested. In short (per my understanding), at 6 and below Adobe uses sub-sampling, basically turning a 2x2 block of pixels into a single chroma value. At 7 and above, each of those 4 values has it's own chroma value. Adobe wanted to keep file size increases roughly linear, but since this change creates 4x the chroma data, it made concessions elsewhere. The conclusion is that you're probably better with 6 if chroma data isn't as important, or something higher than 7 if it is.
To see the differences between the various levels, I took a recent shoot of a home decorated for Christmas. I resized it to my preferred delivery resolution of 2048 pixels, then compressed it at 12 through 1. I then re-loaded all of the JPEGs into PS, and using difference blend mode to see the difference, and a levels adjustment to emphasize changes. Below is the absolute difference between 12 and 1, I included to show that it's not as dire as the below samples seem, but they does give some insight as to changes in the different levels of compression. Look at the change between 6 and 7!
If you want to examine things more on your own, I've included a link to the files here: DropBox
Effects on final images aren't as bad as even these seem. If you look at 12 and 1 side by side it doesn't take a lot to see the artifacts, but at viewing sizes it doesn't turn into a pile of mush either. Personally I'm re-evaluating my delivery compression for clients, but I probably need to do a little more research before I make a decision.