Tallyho IMHO setting the contrast to under 50 is a bit OTT to help prevent screenburn. From the day I got the Samsung and my Panasonic before it, I set the screen up correctly using the Digital Video Essentials DVD and have never had any issues with Screenburn. Now I would advise you to invest £15 in a copy of this DVD as the results are quite staggering, yes you could use the THX set up on say a Star Wars dvd but the DVE disc is far more indepth and it may take a few hours to calibrate your TV correctly but it is worth it trust me! Then every few months re-test the tv for any minor adjustments.
My settings at the moment for normal TV watching are 70 contrast, 48 brightness NO sharpness!! and 48 colour.
As for prolonged 4:3 viewing, if you are worried you can always force the screen to widescreen via the remote.
Anti-screenburn tech is working in the background on your Samsung but can be adjusted on the menu making the screen shift more pixels and more often etc.