instead of taking BOTH on the return leg take off i went for FULLEST didn't fancy a cough from the engine climbing out with 4 pax on board...
You are much more likely to get a cough on takeoff with a single tank selected in a 172 even if the unselected tank is empty. The POH recommendation for using both is there for many reasons, have a read of the POH and familiarise yourself with the fuel system.