In my mothers day, when the ironing was finished she emptied the reservoir. I think this is why her iron lasted 20+ years and a few more with me.
Many irons have plastic reservoir now so I'd think it isn't useful to drain, but many irons with auto shut off, don't actually shut off. They rest at quick-reheat setting and overnight can drain the reservoir, I guess via evaporation. so the water is still going through the system