That was a danger with CRT's because, each time the electron beam sweeps across the phosphorous dot in the screen, it erodes a little bit of the phosphorous. So, if you you often don't use some of the phosphors (view a whole lot of letter box movies and rarely watch full screen program for example) then you will get differential wear on the screen thus burning the image into it.

This is not supposed to be a problem with LCD's. These produce their images through a completely different process. Therefore, they are not subject to that kind of wear.