My buddy drilled a hole in his air tank when we were redoing his setup to see how much water was in it, go about a 1 inch diameter wet spot on the drive way out of it, his truck had been bagged and daily driven for just under a year.
If you wanna play it safe get a water trap, AVS has nice little billet aluminum one for $35 http://www.avsontheweb.com/home.php?cat=322
and AIM has one for $20 http://www.airbagit.com/SearchResults.asp?Search=water+traps