I would check the SG just to make sure it didn't change too much if there was some evaporation. Otherwise, it should be just fine. I use saltwater mixed over a month old and had no bad effects.
Some salt mixes, like Red Sea Pro Salt, precipitate when held over long periods, but as long as you're not making over, say, a 20% percent change, it really shouldn't matter. You can check the Ca and Alk if you're feeling paranoid.