I'll just keep buying good RO water for $3/5USG and not have to even consider water being a problem should I have issues with my plants. Boiling tap water to make distilled is the most expensive way to get a little bit of water and only going to be good for a couple tiny plants. My four flowering girls take a gallon+ each every 4 days. I stopped using my Polar Bear distiller after running a meter on it and then calculating out my energy costs at twice what the RO water cost. Selling that pig off soon and it was $3000 new. Previous owner bought it and I got it when I bought the house.
Rain water is the cheapest but can be acidic if you live on the eastern side of north America because of all the coal fired electric generation that goes on out there. Much better than it was 30 years ago when acid rain was killing lakes and forests all over eastern Canada and legislation was finally brought in to force US power producers to start scrubbing the pollution out of the smoke stacks. Most of the acid was nitrates so at least there was some free fertilizer in it tho the lead and mercury levels were thru the roof.
Hard tap water is the biggest problem for indoor growers with all sorts of crap in it. If you're going to use it then contact your supplier for a copy of their water analysis reports. Should be free. At least with hard water you don't need nor should you add CalMag even with coco as the water will supply more than the plants can use depending on it's composition that you'll find out if you get a report.