It's always a good idea to have at least 2 switches to turn off your auto-top off and even better if those 2 switches are different types. I have a double float valve switch and I also have an optical switch as a last resort. This way I don't have to worry about the age of my float valves. When the first one fails, hopefully the second one is still good. If not, it will hit the optical and I'll know to replace my float valves. I also have an Aquatronica controller, so I will see a decrease in my salinity and will probably be notified by email when it happens, as I'm guessing my salinity will drop enough to trigger an alert.
It's also a good idea to have a sensor in the tank near the overflow to stop your sump pump from overflowing your tank. I will use an optical sensor for that but I haven't hooked mine up yet.