Testing your water regularly is kind of like getting a prostate exam regularly. You're just increasing your chances of finding something that isn't going to hurt you!
I only test regularly because there are natural gas wells in my area.
My house had one when I moved in. My trouble was that after tests, purges, disinfectants and purple water (I'm not kidding!) for several weeks I still couldn't get clean water. After calling out the Culligan man twice (expensive out of town fees), the guy came out, turned a dial and went home. Never did a bit of good.
I finally figured out that I had to rip the bloody thing out of the house, and that solved my problems. My water has been clean and soft ever since! I cannot figure out why it was in the house in the first place.