I just searched "best water for steam irons." There are many theories but the one that makes sense to me is that some irons are now manufactured to use medium hardness water. I have always used tap water with no harm, as far as I know, even when distilled was recommended.

For more years than I can count, I've used distilled water in my irons. A knowledgeable service person suggested it to me because of the extreme hard water that we have in our rural area. I've never had a problem with my iron and usually only change irons because someone in the family needs an iron which gives me a chance to get myself a new one. Five or six years ago we installed a soft water system in our home, but have continued to use distilled water in the irons. I recently watched a Fons & Porter program about caring for your iron and they strongly cautioned about using distilled water. I went on the Sunbeam website and they also discouraged the use of distilled water. Can anyone enlighten me?