I've been using water processed as you describe for about a year now on a few hundred plants. Generally, it seems to be OK. I've had a couple 'babies' fail, but not sure that it was the water since I'm still a neophyte at orchid keeping. Many others have done just fine. I have to admit though that my deflasked sprouts have only seen Reverse Osmosis water after the softener. I also have used a 'K' brand softener for over five years with no problem. The percentage of salt (potassium) that remains in the softened water is very slight, but increases with the hardness of the water you are treating.
The same would be true for sodium, if you used sodium chloride instead of potassium for ion-exchange resin regeneration.The formula for calculating the amount of additional potassium follows: mg of K / quart of softened water = grains of hardness X 7.5 mg K / grain of hardness.