i think lower compression will introduce 'compression artifacts'. It won't lower the contrast, or 'depth' of the image at all.
Yeah, it is a good question though. I had been compressing at level 9 for a long time because that is what Myloupe wants it at. The sizes are drastically smaller than at level 12. However then I realized that some sites state the image size, and thought it is probably wise to give the buyer the largest image size possible. Even if they don't see the image size, the buyer will be happier with the better image they get. So i do everything at quality 12 now, and just wait a few more minutes while ftp'ing.