Screen savers are a trifling relic left over from the days of the CRT display. Since leaving an image up on a CRT for a prolonged time caused burn-in, screen savers were created to "save" your screen from this burn-in; they made images constantly move.
Now, with the proliferation of LCD monitors, screen savers are no longer necessary, and are in fact detrimental to the life of your display. Since LCDs are not subject to burn-in, the only thing a screen saver does is wear down the life of your monitor's backlight.
The best thing to do is simply tell your display to turn itself off before going to sleep instead of using a screen saver.
You can shut off the display or wait for it to turn itself off on its own.
Just press Control+Shift+Eject.