It's difficult to measure the effectiveness of it.
(a) You don't really know what buyers are searching for (you are guessing).
(b) You probably won't tweak a 'good selling' image.
(c) Only way you'd know is if you tweak something that sells nothing (that you think should be selling something), and then see a big spike in sales. But that would be a very time consuming process (especially as it seems it has been made deliberate difficult to tweak keywords once approved, i.e., you have to use your mouse, drag/drop keywords/etc).
So no. I think I did do it 1-2x, but it was so time consuming, and there wasn't a real easy way to see if it made any difference at all, just left it.