The manual notes that viewing conditions like the ones you described shouldn't be more than 5% of the viewing time for the television per week:
What you can do is after watching a movie with the black bars, run the Scrolling Bar feature in the Screen Burn Protection menu to exercise the pixels that were dormant. I wouldn't overuse the feature either, but it allows you to even out the panel in increments so that image retention and burn-in artifacting stays to a minimum.
When I am watching tv it is common for some shows to not fill the whole screen. I have been a little cautions because I have heard about plasma screen burn and don't want this to happen to me. Is it that big of a deal? Also I subscribe to netflix and a lot of the movies I get don't fill up the screen. When I stretch them to fit, the pictures look funny and takes away from having a nice tv in the first place. Is it bad to watch a 2hr movie with black bars on top and bottom? Is it going to ruin my tv? Or is it not that big of a deal?