Well a few years back I did some research on this subject and I work for a company that sells water softeners. I live in the Texas Hill Country and have a well. Our water is some of the hardest in the country, it has 34 GPG. 1 is considered soft and 10 is considered very hard. You really need a Hard Water Test Kit to properly test the hardness. A TDS meter will tell you "total dissolved solids" which tells you how much crap you have in your water but not necessarily the hardness of the water.
A big misconception is that if you have a water softener that uses salt or potassium you will have a lot of salt or potassium in your water. This is not true at all. The salt/potassium is only used to clean the resin during the regeneration process. The resin actually traps the calcium and magnesium (hardness). There is a little salt/potassium released into the water but a quart of soft water will have the same salt content as a slice of bread (not much).
With that said, you probably don't want to use soft water or hard water in your wine. It was recommended to me to use "spring water" from your grocery store. It has a few minerals but is about the best balanced water you can get IMO.