I keep mine on auto. That way, if either the propane or electricity shut off, you are covered.
Think about it this way; how much of a pain is it (and is it cost effective) if you run the fridge on propane, but when the propane runs out, you have to drive and get the bottles refilled? Not only costs of driving there, but the time it takes you to unhook the bottles, load them up, get them filled, loaded in the truck, and then find someone to unload them for you( I am guessing by your name that you are a woman and 30lb propane tanks are heavy)?
I say use the electricity and keep the propane for a back up.