Does it present incredible risks in case of accident ?
Carry the fuel in an approved container, some local laws require color coded tanks for gasoline, diesel, alcohol etc. If you must carry fuel in the interior of the vehicle use common sense. Don't smoke, and open the windows to avoid a build up of fumes/vapors. If you are using the fuel to power a lawnmower, generator, etc carry as little as possible for the shortest distance you can. An alternative is to fill the fuel contaner from your vehicle using an approved siphoning devise. Do not insert a hose in the tank and suck on it with your mouth. If the situation requires carry a portable container, position it in the vehicle so that is unlikely to tip over or slide while the vehicle is moving. If the fuel must be carried due to the distance between fuel supplies then empty it into the tank as soon as possible.If it is 350 miles between supplies and the vehicle range is 300 miles add the carried fuel as soon as you can. Don't carry the fuel in a portable container any longer than needed.
When going on long trips or out of the way trips where you know you may not see a gas station for a while, it's worth bringing along extra fuel in a safe and approved container (ie, don't put fuel in a plastic soda bottle and call it a day).
However, the safest policy, if you're just going about town, is to keep your eye on the fuel gauge and plan accordingly. Things I always maintain in the back though, are antifreeze/coolant, synthetic engine oil, and hose repair tape.