I have always emptied my iron after I get through with it for the day. I suspect that some of the problems people are having with their Rowenta irons is that they don't empty them. All the manuals I have ever read, including the ones for my GE and my Black and Decker say empty them.
My irons-GE and Rowenta at the moment-say to use tap water. They specifically warn against softened water. I use reverse osmosis water and have for years, but would not hesitate to use tap water.
All metal will corrode if left in standing water long enough. My neighbor left water in her iron for several weeks while she was out of town and when she got home, the iron was ruined.
Of course, she left hers flat instead of on its heel, so that might have made a difference.
The chemicals in the water settle and collect in the steam orifices if you leave water in the iron very long. It doesn't take but a minute to empty it, so play it safe and follow the directions.
froggyintexas