For nano tank I will check my salinity every time I do water change.
No room for mistake especially for nano tank.
The parameter will be like roller coaster. If need to any adjustment, do it slowly.
Evaporation don't cause the salinity to drop if u have an ATO.
But skimmer will, Cause skimmer will skim out salt water if u set it to wet skim. This will cause part of the evaporation too.
For bigger tank usually don't make lot of difference because of the water vol.