I wonder what Apple has to say about this?
I've noticed that people offer various personal advice on this, but nobody references the engineers who actually designed and built the screen. I'm assuming Apple did consult the engineers on this issue and they actually took the time to write about it in the "
MacBook Pro Important Product Information Guide." They probably thought it was important or something.
I'll quote page 2 from the 2012 rMBP version:
"
Cleaning the McBook Pro Screen To clean your MacBook Pro screen, first shut down your MacBook Pro and unplug the power adapter. Then dampen the included cleaning cloth with
just water and wipe the screen.
Do not spray liquid directly on the screen." (Emphasis added.)
Coming from a machining background, I would recommend using a swirling motion to avoid wearing the screen in a uniform manner. Take note that this is not explicitly stated in the guide, and is probably not necessary considering the included cloth is so soft that it probably wouldn't affect the screen this way.
Also, coming from a physics background, distilled or filtered water certainly counts as "just water", so that would be fine according to Apple. (Off topic as an aside, drinking distilled water is perfectly fine as it is just purified water; some people I've met seem to think distilled water is somehow bad for humans.

Go figure.)