Sue, something to keep in mind with the hempies, if you're using rainwater you will need to add some sort of Cal mag to the mix. Also I believe most calmag, or at least the version I use, has some nitrogen in it -so I find that it works as a low level nutrient for the first couple weeks and should be accounted when you're mixing your nutrients.
You were asking about ph'ing the plain water. I don't bother when using rainwater. Tap water, maybe/probably- but it depends more on the ppm value I think. For one thing clean rainwater is the right ph, at 5.6. But more importantly the ph reading it gives is pretty much an empty value, and it contains almost nothing but- water. Pure water is neutral at ph 7. The acidity of rainwater is due to carbon it picks up from the atmosphere in its travels making it an extremely weak carbonic acid. The tiniest drop of ph+ or ph- will sway the ph of the rainwater dramatically. This tells us that its ph value is not going to have much effect on the ph levels of your hempy pots. I think your tap water can be judged in the same way. If it's ph value is easily shifted by small amounts of ph+/- then you can assume it's going to have little effect on the plants. Does this make sense?