the reason it is advised that you turn your monitor off before cleaning is because the general public is not very bright, and likes to do things spray windex on a screen until is dripping off of it. By telling people to not use cleaning product and to turn off the monitor it absolves them on responsibility for others ignorant decisions.
Instead of using windex, why don't they just use a bucket of water? I'm sure that would get it clean
I use a slightly damp cloth every once in awhile to clean off my monitor and it still works fine.