In most areas there is no problem using tap water. There are some places where the water is not so good, usually on the acid side and if it overcomes the buffers in the coolant, there can be some problems so using distilled water will not hurt. That said, I have never bothered and I would guess few people do and very few experience any problems by using tap water. It is important to change the coolant as it appears you have been doing. If it has worked for the last 40+ years, I suggest keep doing what you have been doing.