I would disagree. You're going to have to dilute out whatever contaminants remain (and get absorbed into the LR
and substrate) after your cycle. Plus, the cycle start is the time when algae can grab hold, silicates and nitrates in tap water can cause this to become a real problem. RO
water at the store isn't that expensive, 37 cents/gal
@ wal-mart and 39 cents/gal
at grocery store (Culligan water). I have 5 gallon Home Depot buckets that I use. I can fill up on 40 gallons is 20 minutes (and yes, I've done that, twice in a day one time).
An extra couple hours on initial setup can save you days of hassle from using tap water. Start it right, that's my advice.