I'm on well and septic for over 10 years. You most certainly have to use RO/DI. The reason for that is the minerals in the ground are being brought up in your well water and not treated. You want to remove EVERYTHING from the water.
As for the septic end of things. I do things a bit more conservative than I've heard others do. I do not dump any of my saltwater change water down my drains. Septic tanks are made from concrete. Salt eats away concrete. I know, I know, the concentrations from the 10-20 gallons of saltwater at a time being dumped into a 1200 gallon septic tank won't do it any harm, but I don't want the accumulated effect to wreak havoc on my system. Anyway, I just dump it in the street.
As for the RO/DI discharge/waste water......I also do not dump that down the drain. Overloading the system with excessive water is not ideal either. I make about 100 gallons of RO/DI at a time. That equates to about 400 gallons of waste water. I don't want to tax my septic.
As long as you do things with the knowledge of how a well and septic work, you will have no issues.
If you have any questions, please feel free to ask. I was a general contractor for many years, a reefer for many years, and a homeowner for longer than most of the members on this forum have been alive......... yes, I'm almost geriatric. :sad2:
Russ