I usually try to add only enough fuel for what I'm doing so the tank goes empty about when I'm done. I'm not real good at carb rebuilding so I try to forgo this. When I store stuff for a longer period than a couple of weeks, I open the tank cap so no moisture can build up--you are right on about this--(risk here is you get dust/trash in the tank, but I put a fiber filter in place of the cap to prevent--This issue here is I've found the fuel will eat up the fuel lines if left with any gas in the tank--Like I said, I'm not that handy with carb rebuilds and total fuel line replacements, so I try to minimize the loss----I agree-Seafoam is a good additive.