Well, technically LCD screens can still get burn in. You may start to see burn in from the menu-bar icons (or the start menu in Windows), but it isn't such a huge problem and typically isn't permanent. I still use a screen saver because I think they're pretty, but it probably isn't necessary because I play full screen videos all the time.
Turning off the backlight is probably just as good an idea as a screensaver, but either is a fine option.