[Richard]

But can't the water become "contaminated" while in the airlock?  Maybe I am missing something...

I guess if you were fermenting a mead for months that could happen, but for an ale fermenting for a couple of weeks there is no problem.

[ynotbrusum]

OK, but then why worry about boiling the water?  Not trying to nitpick, but just wondering.

[Richard]

OK, but then why worry about boiling the water?  Not trying to nitpick, but just wondering.

Well, some people don't bother boiling and seem to do fine. I like to have water that is pre-boiled and cooled to room temperature ready at hand for rehydrating dry yeast. Since I have it on hand I use it for my airlock, too.

[narvin]

The only real issue is suckback, and it's likely a remote one if your tap water is decent enough.  I used star san before i switched to blow off tubes because it would be effective for a longer time than Iodophor. Bacteria doesn't fly, even in an empty tube.  See: swan neck flask.