Yes... what he said ^^^.

My Mac Portable was left turned on, displaying basically the same image when it wasn't in use for (literally) a couple of years. After a few months, the black pixels would start to remain black - in other words, I'd drag a window and its outline would remain at the old location. Turning the computer off for a day or so, or displaying an all white image would fix the problem.

Nowadays, the Mac Portable is rarely used, but I can find no sign of long term damage to the display.