A good quality refractometer is important to main optimal salinity. There is a wide range of choices for salinity. Many prefer a low salinity for fish only systems, around 1.018. If you plan on having liverock, corals, snails, shrimps, it would be best to keep your salinity a little higher.
The corals generally prefer 1.025, but you can compromise and run anything from 1.020 to 1.025. The most important thing is to keep consistent. You do not necessary have to get the most expensive tester on the market, even the low priced little SeaTest models will work.