I'm currently editing a bunch of images (JPEGs) and Photoshop keeps wanting to save them as 10/12 instead of 12/12.
Why does it constantly want to save it as 10/12 instead of 12/12? Is there an algorithm that says that 10 is enough for this picture (or all of them)?
Or a bit differently: Should I do 10/12 or 12/12? Yes, quality matters.
If you really are sensitive to the quality then you should avoid JPEG. You already lost quality when the original image was saved as JPEG and nothing can bring this quality back. In general, you should avoid saving your documents out to JPEG unless you are shipping the images off somewhere in their final form.
It's hard to say whether the quality suffers much at all, as this depends on what the original JPEG file quality was. In most cases the best quality metric is the same as you started with. Unless you cropped or rotated the image. In theory it is possible to encode each section with the same compression. This is especially true if all you did was color correction or left a portion of the image intact.
Also, there is not much point in saving at a much higher level of quality than the original, as all you get is a bigger file, for a lower compressions quality.