The valve is a must with the ethanol gasoline and your gravity fuel delivery.
Even with stabyl in your fuel there will still be water in the gas. Water is heavier than gas. The water makes NO START problems .
Half machines have fuel pump. The others are gravity forced fuel .
Water WILL get into the carb bowl and make the jets weaken and open up. What happens is the gas passes from the carb to the valves . If the intake valve is open the crankcase fills with gasoline . If the exhaust valve is open the muffler fills with gas .....makes a BIG bang . When the oil is ruin you get the 18hp white fog machine .
I deal with about 2,500 machines over the year .
Full shut off gets added when ever there is room for it in the fuel line .
I have not had to touch a carb in my five tractors and four push mowers since adding the valve.
Just close the valve and let the carb run DRY .
Good luck .