My theory on plasma break-in, is that it won't improve the picture, as every pixel will eventually break-in on it's own. But it does allow for a more consistent picture by getting the phosphors through their first hours of use, where they change the most rapidly, all at the same time.
I did 7 days of slides any time I wasn't watching actual content. After that time I noticed the lack of after-image (one of my calibration DVDs has hard white titles on a dark gray background, it would always after-image on the brand new panel), and less sparkles (but they were not noticeable more than a couple feet away anyway).
