You're quite right, to maximize the life of the cooling system the preference goes de-ionized/distilled water then soft (rain) water then hard (mineralized well) water. It's a bit like single malt vs. 12-year old blended vs. unaged Scotch -they'll all get you drunk, the first is for purists and the last more easily makes for a headache. But I digress.

I only say this so I can add the thought that for water supplies treated with chlorine, if you can run it through a charcoal filter (even one of those little jug filters) to remove some of the chlorine then all the better. Or let it stand overnight to gas off. That will leave a wee bit more of the antifreeze's fighting power.

In any case, regularly changing the coolant is probably more important than the type of water used. Other factors come into play as the coolant ages

With an older cooling system, if you're going to flush and the coolant hasn't been changed in eons then the odds of a water pump, radiator or heater core failure within the next few months are vastly increased. That's why it's often better to skip a water flush and just do a thorough drain if the coolant has been left too long in an older system. That's also why some people put in a new water pump with a new rad. But no one in their right mind would change a heater core as a preventative measure.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now