Does using household softened water hurt outside tomato plants and flowers?

It shouldn't make a huge difference. Softened water has more sodium and less of other kinds of minerals. But the amount of sodium won't be enough to hurt your plants.

Between the two, I would go with well water. Why bother softening the water if it's not good for the plants?

Water is just the carrier of nutrients to your plants. The fertilizer balanced by and made available by the pH of the soil solution is so much more higher in concentration on an exponential level, that softened water is of no consequence. If you can save money not using it, don't. It is not enough to effect a plant. Plants have no taste and love minerals but the amounts in any drinking water in the First World countries are nothing of any consequence.

Well water would be preffered. Just like taking caffeine out of coffee alters the biological make up of the liquid. Taking and softening the water by siltering it with sodium will not be as good as well water.

The main reason though is the taste of the tomatos. What gives flavor to any fruit, of which a tomato technically is, or vegetable, is the mineral content of the soil. Well water adds mineral content and you will have tastier fruit. That should be reason enough.

http://tomatoplanterupsidedown.com/

YES, IT CONTAIN SALTS. Best thing one can do is use a rain barrell. It is filled from the down spouts off your Roof, an get the purest rain wates. If your spouting has litter in it put a 4 inch plastic drin hose on the downspouts exit end an a pair of panty hose on the ed aninto a barrell. This will filter it easy. then just use the collected free mothers nature water an water your plants an Veggies!!!

well water is best, but I want to know whether water softened by the addition of potassium chloride instead of sodium chloride is harmful.

as long as there is not any chemicals fairy liquid or cleaners in it should be fine just water around the plants not on to them