I don't use tap but purified and distilled water mix.
I have always read to mix 5% - 10% tap water with distilled water as by itself its to aggressive for boilers.
I haven't done a exact mix between purified and distilled water (I've always kept both on hand for various reasons). I think when I first joined here I read about straight distilled being bad, so I started to mix. Though just as important, if not more so, is as the thread title says "Empty your boilers after use!".
@Raphael that seemed really fast to get your boiler back! I'd wished I had places to send Empire's and Weeden's LOL