Great question. In general I would say, 'How much fluctuation in salinity are you willing to tolerate?' I have a 150-gallon FOWLR and "no" ATO, and my salinity can drop quite a bit over time before I stop being lazy and top-off. Even with a reef tank, I would think much would depend on the sensitivity of your corals (hardy/cheap corals v. expensive/sensitive corals). You can try your ATO at different levels and test how your salinity changes at each 'step down' or 'step up', and see what level you prefer as a salinity swing. My 2-cents, anyway. I hope this helps.