TBH, the whole NOT ph adjusting the tap water sounds a bit funny to me as well. But IMO, thats not a reason to NOT try plain tap water. I did read up a bit more yesterday regarding NOT PH tap water and its a bit of toss: there are folks who believe that there is no need and there are plenty (like yourself) who sit in contrast.
Really, the only way to be sure is to do it both ways: Same strain (clones), same soil/feeding and the only variable being tap water. Even then, there are assumptions being made (such grow box/tent remains same, growing media has buffering capabilities, tap water from municipal source does not get changed etc).
Now, i plan on doing both eventually but given how the upcoming one will be 2nd grow (cumulative) and i am far from being a seasoned grower, i'll be erring on the side of PH'ing my tap water. The 3rd grow will be tap water with the same strain.