It's only bad if it gets sucked back into the fermenter. The liquid in the airlock can get sucked back in if there is a significant temperature drop. Otherwise tap water is fine.
A half oz of tap water getting into your beer isn't going to hurt anything. Beer is not that delicate or fragile. I use a pump when I transfer from the kettle to the fermenter after chilling, and I don't like leaving all that perfectly made wort inside the hose and pump, so I flush the thing out with tap water until I see water come out the other side into the fermenter. I basically get a couple cups of tap water into my fermenter every single time I brew.
Also, regardless if what liquid is in the airlock, if it gets sucked in due to the vacuum action of a cooling carboy, your airlock is wide open after that, which is potentially a bigger problem that the liquid that went in.
In my airlock, I use what is convenient. That is often star-san, because I've got a bucket of it sitting there from when I just sanitized the carboy, so I'll put it in the airlock.
If I need to top off the airlock on some future day, i just use tap water.