I am unsure how to advise on this. Your water sofener discharges brine when recharging the resin tanks. This is a very soluble salt solution that could contaminate groundwater and affect your water supply. Other than that, it really doesn't matter whether it is in the septic system or other drain system, the salts would not respond to either filtration, settling or anaerobic decomposition in the septic tank, so they are not treated anyway. Both systems will simply percolate them to the ground.
If your groundwater supply is deep and well isolated by clay or rock layers, you are probably OK to discharge in either location. Just keep in mind that salts are not broken down and can migrate quite a ways until they are diluted to low levels, which is why they are a pollutant of concern.