I use a vented cap, so it makes itself the high point in the system more or less. The vent on my old tank was just an AN bung in the top to a piece of fuel hose and a keychain sized PCV filter, the ones that look like mini K&N's. As long as the fuel vapor has a place to go, your fine, just try not to vent it straight into a hot exhaust crossover or something.
This is a picture of a modified setup that a guy with a desert-runner truck made. Its a fuel cell with reducers to the factory Ford filler neck and cap, and he's running a breather as well as the factory charcoal cannistor.
