The salt doesn't evaporate...just the water; as the water evaporates, all the salt is left behind. So, as water evaporates, your SG rises. If you just top off with water (preferably RO or RO/DI), your SG will remain the same. The only time you really need to worry about matching SG is when you do water changes.
Some people add kalkwasser to the water they use to top off the water, to replace the calcium that might've been used up by the inverts and fish, also liverocks.