I've only done 4 so far, but they've all been topped off with tap water so I don't see an issue. I don't pre-chill my water though, but it's about 58 deg out of the tap (out town taps a 600ft well). I do let it run for a minute or so before I top off, but other than that, I figure if there's enough bad stuff to kill my beer, I'd be dead by now. One caveat is that I usually end up with about 3 1/2 gallons after the boil. I'll dump tap water in the pot right at flameout to fill it about 3 inches below the rim, give it a gentle stir and then go grab a beer. I'm guessing I only add about 3/4 gallon of "unpure" water since the first topup is at boiling temp, and the 3/4 or so is after I've transferred to the fermenter. I like to get as diluted a wort out of my pot as I can so I don't lose as much fermentables when I try to keep the trub out....but sometimes I just dump it in if I'm lazy.