Actually, nothing shelters the fuel from oxidation if air is present. Gasoline will go bum-stale after a month or so, so make sure you use that stuff up! A $2 bottle of fuel stabilizer can be bought at your local hardware store (which treat many liters of fuel), and it will keep the fuel fresh for up to 2 years (so they say). This is what I put in my car when I go to store it, and in the spring it fires up fine. Before I used stabilizer, I would have to empty the carb of all that oxidized crud in there before it would fire.
The pressure in the tank is a positive pressure (above atmospheric pressure, in the tank) whick is good. The last thing you want is a tank whick is sealed so tight as to cause a vaccuum. Then not even Hercules' fuel pump could pull the fuel through. Seriously, if you ever think your fuel cap vent-holes are bunged, then stick the cap against the wall of the self-wash and spray the hell out of it. Minor dust can plug a caps vents easily.