Give you some perspective:
I have a Sampo plasma. Great deal when I got it 3 years ago at $3K, but it was cheap junk then, and it is total garbage today... when it comes to image quality. Absolutely ZERO burn-in protection and I can see minor image retention everytime I have some high contrast image on screen for a bit... like the intro PS3 screens.
Yet, here I am 3+ years later and there is still no burn-in on the display. I would say that I sometimes watch 3 or 4 hours of 2.35:1 movies on the display and then go back to full screen 16:9 stuff... or stretched 4:3 and when the display goes off, there are no borders or burn-in at all.
With today's TVs and their advanced burn-in protection allocations, you should have no issues whatsoever. But, I would agree that switching things up after a movie or two is good form. The best thing to do is always have it in your head that burn-in is possible on plasma/CRT displays and that mindset will help you remember to switch things up and get years and years out of the display without issue.
