In the UK, you don't have to boil tap water - it is guaranteed as drinking water here - but I prefer the taste. For plants, boiling removes some elements like calcium (this depends on the geology where your drinking water comes from, we have a lot of chalk substrate here) and that helps to prevent mineral deposits on the plant pots. I don't think deposits are harmful and you may prefer the look of them (weathered).
I have a feeling that the pH of rain water is often different to tap water. In the UK, rain water is often softer and tap water harder.
