My advice is to follow the advice in the owner's manual that came with your set. If it says to observe a breakin period - do it. If not, don't worry about it.
I have a Pioneer PDP-5070 and its user's manual does not recommend going through any sort of 100-200 hrs break-in process. Think about it: if you were a manufacturer of expensive TVs (and wanted to minimize support/complaint calls and warranty requests) would you skip putting in the manual a critical aspect of operating the TV and instead assume that eventually all owners will go to avsforum to find out your "true" recommendations as a manufacturer? I'd be surprised if even a tenth of all plasma purchasers have spend any time reading a site like this one.
Ok, ok, so the manual does give the usual disclaimer about avoiding static images for "too long" (but incidentally, I have yet to see a definition of how long too long actually is.) Here's my experience though...
After I got my Pio5070, for the first few days I treated it with kid gloves and checked regularly for IR (image retention). Then maybe 3 days in I decided what could it hurt to watch one 2.35 movie? Did that, no IR. Not long after, did a bit of gaming. No IR. Watched some 4:3 std def TV. No IR. Not even a little. All this, before the 25 hour mark.
I'm somewhere in the neighbourhood of 200-300 hrs now. For most of that time, I've had my contrast set a little *higher* than the manufacturer default settings. Looking back I have pretty much watched whatever I wanted. I've played video games more/less right from the start, 3:4 std def content, 2.35 ratio movies. (Although, the 5070 has a nice stretch mode for 3:4 which I like better than black bars.) In the last few weeks, I have logged ~20 hours or more on Crackdown for the xbox 360 - which has a static heads up display. Still no IR!
I do tend to watch a fair amount of stuff that is full screen. And my personal viewing habits are such that I tend not to watch the same thing for more than a few hours at a time. Still, given my experience with the Pio5070 I think all this talk about IR and breakin periods just creates alot of fear, uncertainty and stress for nothing!
(Oh and based on peoples posts on this forum, there is little reason to think that the Panasonic brands are any less robust.)