This discussion is the first I've heard of trying to supply plants with minerals in regard to the source of water. That's usually done with fertilizer or some kind of compost tea.
I avoid using my tap water for plants (except occasionally while inside for winter, I can't afford that much distilled) because it has chloramine, fluoride, and a lot of lime, none of which can evaporate. Many plants can become chlorotic from prolonged exposure to fluoride. Not all tap water is the same. If plants like it, there's nothing to worry about. I've seen a drastic improvement in all plants since trying to use as much rain as possible instead of tap.
If I lived where rain was too polluted for plants, I'd be more concerned about moving to where I can breathe than anything going on with my plants.
