Well, in my opinion, just seeing some grit and water drops sitting down in the neck on the first flap, then you jamb the fuel nozzle in there dropping the grit and water drops in and past the second flap, then flush them down with the new fuel, there's no way that these are going out the small vent tube, which if I remember right come up from the top of the tank anyway. For a few buck, I'll feel better idiot proofing the issue with a cap. I see water in there every time I wash the truck, and that is before I intentionally open the door and rinse out the dust accumulation in the filler area. I'm sure in 99% of the trucks out there it won't ever amount to enough contamination to ever be known, but why not be on the extreme safe side rather than be wondering? Especially if you like to wash the truck and rinse in there once in a while.