I had heard that about ammonia before. I understand that it depends on the type of airbrush and whether there are rubber seals in the brush. Neither of my badgers, nor my old no name airbrush have those seals.
The main guy at Badger says he uses ammonia based cleaners: http://youtu.be/tsW-vN0_lHw?t=58m15s
Also, here is an older post on airbrushing:
http://www.ordofanaticus.com/index.php?/topic/19387-90-minutes-of-badger-airbrush-knowledge/?hl=%2Bbadger+%2Bairbrush