I've not had any burn-in on any of our machines with LCD screens (both portable & desktop) many of which have been on for quite a number of years now. Nor have I seen it on other people's system.
One thing I do notice on some screens is kind of a paler horizontal line around one or more, or all of the edges of monitors. Mainly cheaper pc lcd panels. If I look really close at my 20" Studio Display while the screen is near black with flurry saver on, I can see a very very faint thin purplish glow line around the top edge.
My personal screen settings are half brightness and screensavers are set to come on in a matter of minutes or just fade to black via energy saver.