If you keep the set in standard mode, you should never have to worry about burn in. Yo can worry even less so if you use Avia and or Digital Video Essentials and even less if you get an ISF calibration.
I have seen Panasonics with a still image in straight up, maxed out, vivid mode for over 6+ hours that never had a burn in issue. They had some image retention for about 15 minutes but nothing permanent.
In all reality, one would have to intentionally try to burn in a set. The reason a manufacturer will not warranty against it is that specific reason, intentional burn in.
Most speaker manufacturers will not warranty burn voice coils due to abuse, the same premise follows here.