I thought that burn-in is a result of uneven phosphor wear between the burned-in images and the rest of the screen. But that's semantics.
The Break-in DVD ideas come straight from the Panasonic White Paper recommending against ANY side or top/bottom bars during the first 100 hours of panel life.
That is VERY cumbersome to do even if you watch HD only (most commercials are 4:3).
So, running the Break-in DVD at night and when you're at work allows you to hit the 100 hours quickly and enjoy varied programming earlier (HD, DVDs in OAR and some SD thrown in for good measure).
I intend to mix them up pretty good, but I will NOT distort my widescreen DVDs to fill my screen, or stretch SD content.
The latest Panny looks like it is pretty resistant to burn-in, anyway, so I don't worry.