Impurities in the salt mix would really be the only thing that could settle out. By definition, saltwater is a solution and doesn't settle out until enough water evaporates that the maximum concentration is reached and the water can hold no more (i.e. salt creep, etc). For a tank to go that low, it would literally have to evaporate most of its content. Furthermore, even with no flow, on a molecular level, there is enough kinetic energy to keep molecules bouncing around, so technically water is never perfectly still.
Referencing Budman's question, you can store water in that fashion. However, you will want to uncover and stir it up vigorously before use. Stagnant water will be low in 02 and high in CO2 (and low pH) which, if introduced directly to the tank, can have some negative impacts.