I use tap water and rainwater only when it rains on them ;-). I have measured my solution before and again recently since Vladimir commented that the pH of tap water can vary. I don't measure each time though...periodically.

"Normal" rain water has a pH of 5.6 because as that water travels through the atmosphere it picks up stuff that might be considered pollutants.

Acid rain has pH of 4: https://www3.epa.gov/acidrain/education/site_students/phscale.html

I use rain water exclusively. Due to our business we always have an over abundance of 55-gal. black plastic open top barrels available for the taking. I have them placed side by side around two large barns on our property (perhaps 150+-). I am not concerned in the least about pH, therefore I have never bothered to check. - Silica

Silica, why aren't you concerned about pH?

I use tap water that has been sitting around in jugs for a day or 2. My tap water is 7.8 and when the chlorine is gone it is probably around 7 or lower. I just water. If my leaves show of Chlorosis then my water needs a couple of tablespoons of vinegar added per gallon.