I should know better than to go against what Neale posts, I feel lucky today.
I totally believe in starting with plain dechlorinated tap water with any fresh water species I keep, this eliminates variables should any problems arise. I might have been breeding mollys instead of platys this summer if a buddy of mine hadn't started breeding them first. He is breeding them just fine in plain tap, pH 7.7, gH 180's, tds around 200. He might be one of the lucky half, but the same could be said for many species.
I also believe fresh water with no additives other than dechlorinater if needed is the best route to go with most all commonly found freshwater fish. If you have any sort of problem take it from there, there are plenty of products on the market, some useful, as the marine salt you mentioned, many are plain junk. I just find it odd that all the folks I know & hang out with who do or have bred mollys have had no reason to add salt, marine or otherwise.
The biggest issue I have is telling a newer aquarist to start adding things to the water. I wouldn't suggest this unless absolutely needed, this can make consistent water in the tank difficult to maintain.
Worry; To afflict with mental distress or agitation : make anxious
I don't worry about my water, or most any other thing, I'm not the worrying sort, it is totally counter productive. I completely agree with you on the point of knowing everything you can about your water supplier, keeping tabs on what may affect your water supply, and taking the appropriate measures. Water changes & appropriate diet go without saying. As far as choosing fish according to your water supply, most commonly found fish will acclimate quite easily. This includes mollys. I wouldn't be breeding angels if I went by what is considered ideal water for angels, but breed they do.
Worrying does not even enter into the picture, generally when people are knowledgeable about something they tend not to worry. I think the trick here is passing along knowledge, eliminating any sort of worry.