If your doing an actual water change not just top off then you want to Match the salinity of the tank.
Your tank evaporates water..when doing so the salt stays in the tank and increases the salinity of your remaining water. That's why you too off with regular Ro
water so the salinity is then back to what it was.
When you do a water change...in taking out x gallons you are removing the water and salt it contains. So you want to replace that water with the same amount of salt to maintain that balance.
My routine is fill a 5 gal
bucked with Ro
water, add salt. And place a heater and power head in it for a day or 2 to mix and heat the water to the same as the tank. Then I check salinity. Empty 5 gallons and then add the new 5 gallons.