Mark,
The only time my air vents are open is when the motor is running. IF I can  
remember, I close the air vent as soon as the motor is shut off, especially if 
I  am sailing. A good thirty degree heel can cause gas to exit out the vent.
What happens when you leave the vent open is that as the air outside cools  
and the gas cools, moist air is drawn into the tank and the moisture condenses  
inside. This is not good because gasoline is lighter than water so the water  
sinks to the bottom right where the gas pickup is located. Gasoline can be  
compressed where water cannot.
 
Rummy