This spring I disassembled an engine from a 1965 MGB that has been in the family for the last 30 years. It has always been filled with a 50/50 mixture of Prestone and tap water. There was virtually no rust anywhere in the cooling system. Now I am hearing that you should only use distilled or de-ionized water. Based on my experience, it would be a needless complication, although rather minor in cost and nuisance. By the way, as far as I know, the MG still has the original radiator.
Also, if you go to the Prestone web site, they show a radiator being filled by a garden hose.
Is there any factual information that would cause one to believe that the use of tap water is harmful?