I use vodka. I think tap water runs the risk of contamination, espescially if temp fluctuations cause some of the water to be sucked into the wort. I dont know what the level of risk is however. I would err on the side of caution and use vodka. I worry about sanitizer-laced water negatively affecting the yeasties if some hits the wort, although I have read that this shouldnt be a problem.
Someone who has been doing this longer please chime in and correct where necessary.
I use either vodka or Star San sanitizer, but if you use unboiled tap water for the brew, then tap water would probably be okay for the airlock. One thing I like about sanitizer is that if something somehow falls/floats into the airlock, if it gets sucked into the beer at least it has been sanitized.