I've used it last in February and may only need it again in December. So, if I don't put the additive in now, am I going to have to drain all of the gas before using it again? In other words, does the gasoline go bad in 10 months or is it more to prevent losing too much of it through evaporation?
Gasoline deteriorates over time by the way of evaporating volatile components and oxidation that creates gunk which clogs up the fuel system. It is actually better to add the preservative (fuel stabilizer) than to drain it because all rubber components in the fuel system - hoses, gaskets will dry-rot and crack. So, yes, add the fuel stabilizer before putting the snow blower away for the summer.