Viewing Preference?

For those with a 16:9 monitor... 1 - when you view a 4:3 source for an extended time (i.e. typical movie length 2 hours) do you prferer it stretched or normal size with the side bars? 2 - if you use 4:3, is it better to avoid burn in to use gray or black side bars?
Also, thanks to this forum, I found out how to set the output of my Comcast HD box.