In my own personal experiance, I find that the Tamiya thinners thin the paint down better then using just alcohol, or distilled water, windshield washer fluid etc, etc.
I used the others I noted above, but my paint jobs took a real turn for the better when I started using Tamiya's thinnner with their paints.
Mind you I don't use the Tamiya thinner to clean the airbrush afterwards, I leave that to the cheaper stuff like windsheild washer fluid and such.