Remember that both hard and soft water have minerals that will ultimately be deposited in the soil. Most plants don’t take to soft water, as sodium is more difficult for a plant to deal with than magnesium and calcium (the hardness ions). Rain, distilled, or deionized water is the healthiest for plants because they generally do not contain elements that are difficult for plants to use.
We recommend leaving your outside hose bibbs tapped to the hard water line as watering your garden with soft water is both wasteful and more difficult for plant survival. On the other hand, other applications such as car washing persuade some consumers to turn one hose bibb soft and leave the other hard.