I've also been interested in this question, and was looking for a "horse's mouth" answer. It would seen that ANY standard water is OK for your car, be it tap, distilled, de-ionized, or softened. I say that because many authoritative websites (AC Delco, Prestone, and others) don't specify a certain type of water in their car care websites covering coolant service -- they simply say "water." That said, there's "acceptable" water, and there's "better" water. The quotes below address the better choices. Also, there seems to be some nomenclature confusion regarding "soft" water (such as rain water since it has little calcium and magnesium), and "softened" water (such as Culligan) in which the calcium and magnesium ions are swapped with sodium ions. Since my home has a water softener, I was curious about the specific effects of that.
From Castrol's website: "Distilled water should be used in the formula, as it is free of the minerals in tap water that could denature the coolant mixture and prevent it from properly doing its job."
http://www.castrolusa.com/expert/ae_article.asp?article_id=8§ion_id=2
From Performance Radiator, a nationwide radiator shop chain: "Use distilled water, de-ionized water, or as a last resort soft water."
http://216.239.41.104/search?q=cache:lY4o0beebKsJ:www.performanceradiator.com/Publications/Information/installer%2520flyer%2520final2.pdf+radiator+coolant+water+soft&hl=en&ie=UTF-8
From the European manufacturer of Texaco-branded coolant: "...the use of soft water is preferred for dilution. Though, lab testing has shown that acceptable corrosion results are still obtained with water of 20°dH, containing up to 500 ppm chlorides and 500 ppm sulfates."
http://www.arteco-coolants.com/pils/pil hav XLC-PG.htm
From the Transport & Construction magazine, South Africa: "Most tap water contains calcium and magnesium, which may cause scaling or deposits that form on cooling-system parts such as thermostats or water pumps. Tap water also has chlorides and sulfates, which may corrode metal parts in an engine or cooling system. Use de-ionized "soft water," which can be purchased at most local bottled water companies."
http://www.transportandconstruction.co.za/articles/article5.html
From Recochem, a Canadian coolant manufacturer: "Q: Why do the directions state that distilled, deionized, or soft water should be used to blend coolant? A: Some coolants are sensitive to hard water, which can cause some corrosion inhibitors to form insoluble salts in the water. The result is premature coolant failure. Hard water salts will deposit on hot areas within the coolant system creating insulating films, which contribute to overheating problems in the coolant system."
http://www.recochem.com/cdnav/cdproalfaq.htm
From FleetWatch magazine: "Water with a PH factor of 3 is classed as soft water or acidic. This leads to rust formation, pitting in water jackets and the promotion of chemical reactions between various metals. Water with a PH factor of 11 is classed as hard water or alkaline. This leads to the formation of scale, retardation of cooling, the blocking of tubes and passages and creates hot spots in the engine. A PH rating of 6,8 to 7,2 is regarded as ideal. This is neutral water.
http://www.fleetwatch.co.za/magazines/sept99/002.htm
Lastly, this ASA Shop Tip appears to answer my Culligan water softener question: "If you use hard water in these systems, you are asking for mineral buildup problems. Softened water, on the other hand, contains dissolved salts that act as an electrolyte, encouraging galvanic action between the dissimilar metals. That's why distilled water is quickly becoming the water of choice!"
http://www.asashop.org/autoinc/october/techtips.htm
As for "people" minerals, magnesium and calcium are the two minerals which are too bulky to put 100% RDA into a multivitamin/mineral. So a Centrum or One-a-Day pill can easily have 100% of all necessary vitamins/minerals -- with the exception of mag and cal. Attempting so would result in a "horse pill." Therefore, separate calcium and magnesium supplements are beneficial. But if your drinking water is "hard," it most likely has sufficient magnesium for you (per a medical guide I have). Not sure about calcium, but cheap calcium carbonate antacid tablets easily solve that problem. Removing minerals in your drinking water (through whatever means) is not a good idea unless you make up for it with supplements of some kind. And drinking softened water (Culligan, etc.) can lead to elevated sodium intake (per the medical guide), so install a pre-softener tap and drink from that, or drink bottled DRINKING water (not distilled or de-ionized).