Two proven science facts about plasma:
A) As plasma TV ages, the color temperature and brightness will change, normally decrease.
B) The first 100~200 hours is when the TV is most susceptible to image retention.
So from there, you can think about how the break in, slides or calibration works.
As people mentioned above, breaking in TV with slides is simply an carefree alternative to watching it with full screen content without static logos if you are not watching it. It doesn't improve PQ, nor guarantee the same calibration result even if you run them straight and apply the same setting because not every panel is created the same, not everybody calibrates the TV under the exact same controlled conditions.
For example, when CNET does their TV benchmark, they usually only break in TV for 24 ~ 48 hours before the calibration. Then you will see their parameter setting is different from other people's calibrated setting. Also based on fact A), even on the same TV set, calibration at 200 hours may be different to calibration at 2000 hours usage.
Conclusion? Running slides and copy other people's setting doesn't guarantee accuracy.
However, those settings do provide a general reference as usually you can see what parameters are changed in which direction. But changing the numbers -2 or +2 won't actually make any different to naked eyes, especially on a -100 to +100 scale.
At last, it goes back to the more meta question, what is the best PQ and enjoyable PQ?
Objectively, you can indeed get most accurate color or grey scale in measurement by profession meters and equipments. That accuracy is objective and scientific.
However, subjectively. people have different perception of PQ and viewing preference. Some people prioritize deep dark color more while some other people enjoy vibrate colors. It's possible that an objectively accurate set may not be the kind of setting you like. It's just the same as the forever objective vs. subjective debate among audiophiles. A nicely measured speaker maybe not appease to your ears because it's not as "fun" or "musical" as a speaker measured less accurate.
So what's the takes?
Break in for plasma TV is more about preventing IR in the first 100 hours rather than achieving the best PQ. You can do that with any full screen content and slides is one of them.
Calibration provides a general direction and good reference for TV performance but it won't "improve" or change your perceptible viewing experience.