Ive found even if you have good water, the PH will fluctuate during different parts of the year. At least with my water, which is city water. My tap usually runs from 20-30 PPM. Thats why i use it. But during the winter, the PH of my water is around 8.0-8.5 and during the summer it goes down to about 6.5 .. not really sure why. I check my PPMs all the time and it does not fluctuate very much. So i think either way, if you dont know the PH of your water, it is important to check it at least once in a while. Just how ive always done it.
Wolf