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.