Depends!!!
If you skim very wet: (skimate looks like green tea)
You need to monitor the salt levels in your tank as some saltwater is being pulled out of the tank. If you are just replacing it with RO water the specific gravity will go down.
If you skim dry: (skimate is a dark nasty sludge)
Then just top off as not enough salt is being pulled out of the tank
3-4 gallons of skimmate removed from my tanks a week won't change the salinity, at least not what a refractometer can read. I check my salinity when I do water changes and adjust the new water to compensate accordingly.