If by softened tapwater you mean water passed through an ion exchange unit which trades Ca+ and Mg+ ions for Na+ ions, you would likely be better off using the water before it enters the system. For plants trading Ca+ and Mg+ ions for Na+ ions is generally not a good trade And are you sure the water actually goes through the system? More often than not outdoor hose bibs on houses originally built with such systems are plumbed to get their water pre-softener.I have maybe a hundred CP's or a little better in a dozen or two various sized containers, many of which freely drain. I spend .39 cents per gallon and use 5 - 10 gallons per week for normal watering. That is much much more when repotting or potting new plants. I purchase R/O from the blue booths at Walmart and other grocery store in the area. Where I am going currently the water tests at 6ppm TDS.
EDIT: My other plants get watered from "softened" tap water which tests around 860ppm TDS. Sorry other plants!
If by softened tapwater you mean water passed through an ion exchange unit which trades Ca+ and Mg+ ions for Na+ ions, you would likely be better off using the water before it enters the system. For plants trading Ca+ and Mg+ ions for Na+ ions is generally not a good trade And are you sure the water actually goes through the system? More often than not outdoor hose bibs on houses originally built with such systems are plumbed to get their water pre-softener.