I'm on septic also, i asked this question a few years ago,at many other places also,,,,
answers were pretty much was,, no problem,,,,,,,,
if all you did was put SW in the system,,,yes you might have a problem,,,,,think a lot of us use our systems for more things....
i just have a total ,new system installed two years ago , and i asked the guys what they thought,,,,,they said one cup of bleach does more harm than 55 gallons of saltwater,,,,,
i have a 1000 g tank, and the field lines +overflow pipe,,,
i remember while i had originally asked your same question, there was a issue of salt (ground) build up in areas with a lot of clay for soil make up,,,do a search on "Septic Seep" and see what is said about this,,,,
remember,,,like my septic guy said, anyone selling stuff for your system, surely is gonna tell you to use their stuff
BTW: i had my system checked after 2 1/2 years,,,,was told ,and shown, no problems yet,,everything from the house ,minus downspout water, goes into my septic,,,