"A bit alarmist, if you will permit me. A full fuel tank is much safer than one that is, say, 2/3's or less full. Liquid gasoline doesn't burn, at least not at "normal" temperatures. It is gasoline vapor which is so (very) flammable. Very few car crashes, even those where the fuel tank is ruptured, result in a post crash fire. Use an approved container, store it outside the passenger compartment and be careful, not fearful."

That's your opinion, but I certainly don't take any comfort from it. The possibility of fire is present any time external fuel cans are opened, refilled, or used to refill the main tank. People can and have incinerated themselves from generated sparks when refilling the main tank from a spare can, as well as causing fires at filling stations. Not to mention what happens when a external fuel can comes free in a collision, ruptures, and spills its contents over the accident scene. Factory tanks are (usually) designed to be resistant to rupturing and leaks after a collision, but DOT-approved external fuel cans are not (I have found many so-called 'approved' containers start leaking after their first off-road trip). What risk you choose to assume is your own business, what you impose on your fellow motorists is not. Since there isn't really any reason, 90% of the time, to carry filled gas cans for long distances on paved highways, best to leave them empty until you're ready to head off-pavement.