A little story from me about bottled water...
Bought 5 gallons at Walmart for a water change. Was listed as being filtered by RO and/or DI. Mixed it with salt and did my water change. Had about 4 cups of water left...
The next day, my tank was cloudy and my livestock dead.
Before the water change, my params were 0 on the bad stuff across the board.
The day after when the water was cloudy, I checked it. I had 2.5 ppm ammonia, high trites and trates.
Took the water left over that hadn't been mixed to a friend who is a lab tech with the local water works. He tested it and the water had 7.5ppm Ammonia, 25 ppm Nitrites and 15 ppm Nitrates.
With results in hand, I went to Walmart and spoke with the manager. They took care of us. But it was the last time I bought drinking water from them and I will never go without RO/DI on hand for as long as I own a reef tank.
I concur with the thought that it is cheap insurance. It takes just 1 bad batch or water that you have no control over to make a mess of things.