You definitely don't need to add any salt unless the SWG stops making chlorine and says to add some salt. The salt test is just meant to be a sanity check from ti e to time because as SWG cells get old they report larger and larger differences from reality and you don't want to end up with the SWG reporting 3400 ppm but you really have 5000 ppm in the pool. This isn't a +/- a few hundred checkpoint, more like +/- 800-1000 checkpoint. Salt level is not that critical. For my SWG the manual says 2800-4500 ppm salt is fine, with 3200-3400 being ideal I think.
Be careful when water temps drop below 65 or so because the SWG will report false salt errors and/or stop making chlorine. I found that out the hard way in year one and ended up with salt above 4500 ppm. I don't close our pool and I switch to bleach as soon as the chlorine level starts to drop or the SWG starts acting weird. During non-swim season I use bleach to keep the chlorine level right at 10 ppm, 15 if I'm not going to be able to add bleach for a while.