I believe another proper way to do it is to weld a bung into the tank itself to accept a rollover-vent valve. They are cheap, from Summit and others. If you just leave the gasket off the cap you may get splashing/mist of petrol up on the windscreen, and if you have a rollover, you will be slipping gas out for a fire hazard. The rollover valves allow air in as fuel is used, prevents splashing out and keeps fuel leakage minimized in case of a rollover.
Jaz 834-108-06 - Jaz Products Fuel Cell Tip Over Valves - Overview - SummitRacing.com
This is what I have used.