Another reason to preferably keep it off is related to the CCFL backlight used in displays. They have a rated lifetime - though mostly like 50,000 hours or so (which is in fact quite a lot) - beyond that the light will die/fade and these backlights arent user-replaceable.
Turning things on and off will always have some effect on electronics, but nothing that can be too serious. The way i see it pixels shouldnt be affected, since they are just liquid crystals that change orientation based on the transistors driving them. When you turn them on/off they are merely being re-oriented which happens *hundreds* of times a second anyway.
