All my water goes through a water softener. Like you, I have no bypass valve. The water softener runs on no rust stain salt.
My Kh/GH is zero but my ph is high and stable. I use GH boost to buffer my GH and add additional magnesium (espom salt) with each water change.
All my plants are doing great - sword, crypts, tiger lily, java fern, val's and the floaters. I recentally added stem plants and they are taking off.
I did buy a test kit for kh/gh to keep an eye on things but I find the plants tell me when things are off. They recently were showing magnesium deficiency and thus I add the magnesium. My snails were showing calcium deficiency and I added supplemental calcium.
I'm a newbie so I don't know if this is necessary...but I would think a test kit would be informative. Then watch your tank and see if deficiencies show up.