Short answer is, it depends. If your well water is soft enough you might not have an issue with unfiltered. Otherwise RO is the way to go. With unfiltered water there's a significant risk of a buildup of minerals.
That is also dependent on the type of minerals that are in your well water. A properly made and balanced soil can surprise you with the kind of water it can take and there have been studies to show that the idea of chlorine killing microbes in your soil as a complete fallacy. However, deno is still correct in the sense that it does depend entirely on the specific minerals that are in your particular well. Different wells are going to have different minerals and such in them, so you have to know what's in your water for sure.. but you'll be surprised at what a good soil can handle.
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https://www.dudegrows.com/tapwatermicrobes/"
We're talking about a population of billions upon billions of microbes, and the growth of bacteria/microbes is exponential. So while the chlorine in the water will in fact kill some microbes, it isn't nearly as deadly as people think it is. This is why running RO water rarely fixes people's problems.
I'm using well water, anywhere between 419-460ppms and has so much calcium in it that I have to replace parts in my swamp cooler at least 2-3 times a year. My plants don't seem to notice it and I've never had a problem using my water with a single plant I've grown, not just cannabis. Even when I was in SoCal and my water had over 500ppms I still didn't experience any issues.
HTH