It's my basic understanding that a water softener removes dissolved calcium and magnesium and replaces it with sodium. It's not clear to me, however, which is worse for the reverse osmosis filter, hard water or salty water. The RO system is all plastic, so I'm not worried about corrosion, I'm just curious of the impact on the life of the membrane and the quailty of the water. It's a 3-stage filter system.
My kitchen cold water is not softened (which is typical). I guess people prefer the taste of hard water to softened water. But the rest of the water in the house (except exterior spigots) is softened. I intend to use the RO-filtered water in the kitchen. I will install a separate drinking water faucet for the kitchen sink and use RO-treated water for the refrigerator/ice-cube maker.