Under normal conditions soft water is perfectly safe for plants. Hardness levels in WNC are rarely high enough to cause problems. When treating water that is reasonably hard (less than 15 gpg) it is unlikely that the sodium concentration would be a hazard to plants. Most household plants are far more dependent on soil conditions for healthy growth then the slight amount of sodium present in softened water. If there is a very high hardness concentration in the water being softened, then there can be some concern about the higher sodium concentration of the softened water and it potential to be harmful to plants.
For outside sprinkling purposes, the use of softened water, for economy reasons, is not recommended unless necessary to prevent iron stains on buildings and concrete. Again, where the concentration of hardness minerals is heavy, the sodium salts replacing them might be sufficient to retard growth or kill grass.