In the section on aquaponics Max indicates that his strategy for his flush composting toilet is to wash the hands first in well water which switched the system over to grey water for the subsequent flush. He then indicates that an upgrade is to use rainwater instead if the well water for the hand washing so as not to waste the well water. In watching Geoff Lawton he reacts to this prejudice against rain water with humor. This is because unless there is a lot of contamination (which there should not be) the rain water is essentially distilled water and the well water is distilled water with dissolved solids from the ground. That is to say that the rainwater us the more precious resource and is the cleaner of the two. The dialogue from Geoff also shows that this is not an unusual point of view (that well water is better) even though it is not aligned with the true purity of the liquid.
It is up to each person to test their rainwater but if the pH is about neutral you should be ok.