The problem with storing combustible gases below, especially those heavier than air, is that they may leak. The containers are not very robust. If they leak, the gas builds up in the cabin or the bilge, and with one spark, it does what a combustible gas is supposed to do: Blow you to kingdom come.
So "proper" safety precautions include using a gas detector, a blower, and storing the gasses in a properly sealed and vented locker, so that if there are any leaks the gas vents overboard.
I've seen milk crates full of butane and propane cans in engine compartments, rode lockers and lying on the dashboards under windshields with the sun beating down on them. People seem to forget that these make very effective bombs.