Why the worry about the pH of water? Aren't you worried more about it's buffering capabilities? That would be determined by the overall hardness, mainly bicarbonates, which are the main source of alkalinity and buffering capabilities. Distilled water often shows a pH below 7, but has zero buffering and moves as soon as any ions are added or removed, tap water doesn't move as quickly as distilled if it has any hardness.
It's seems to be a distraction and not important, it may be a misleading data point. The main drivers for pH in beer are the grain, water salts, buffering capabilities of the water and yeast. Measuring the pH of water really isn't necessary because it doesn't tell you anything you don't already know.