No, not with RO/DI water or if the LFS used RO/DIto make their saltwater. Before buying more premixed saltwater from the LFS ask to see their RO/DI TDS readings, the brand of sat they use and the final specific gravity or salinity of the mix. Some LFS are a little lazy when it comes to wate rquality and cut corners.
As already mentioned, you will need jug of RO/DI for top off due to evaporation so make sure it is 0 TDS before purchasing it. If you can't get good RO/D then consider gallon jugs of purple cap distilled water from WalMart, the general concensus is it is pretty good quality.