As being said before, C172s are always flown with fuel selector in BOTH position, unless the fuel imbalance indicated by fuel quantity gauges requires balancing the fuel. I also don't recommend using OFF switch EVER (unless the aircraft is being stored for a longer period - say a month or longer).
Regarding the "flying with one tank empty" topic: is it really safe to let one fuel tank dry out? Isn't fuel system on most aircraft designed with redundancy in mind shall there be a vapour lock (average C172 doesn't have aux fuel pump) or any kind of blockage in fuel line?
I kind of miss the PA38's fuel system in most aircraft - L/R/OFF fuel selector colored red placed in the middle of the panel - it does its work perfectly and if taught properly, it's almost impossible to forget to change the fuel tanks. And also, no complications about "slushing fuel", refuelling and other problems developed by C172's fuel system...