A softener does not add salt to the water. They add sodium at the rate of 7.85 mg/l per grain per gallon of ion exchange. And if you read some food label to compare that amount to the sodium found in most foods and all waters, you'll usually find that isn't isn't enough to be concerned about.
Let's say you have 15 gpg hard water, * 7.85 = 117.75 mg per a quart+ is less than the sodium content in a slice of white bread. Serving sizes of many foods have much more sodium than a slice of white bread.