If you do this, definately secure the can(s) very well. A bungee cord won't cut it. In an accident, a lose can inside the vehicle would be very bad news.

If you had to evacuate, like the famous hurricane traffic jams, you definately want to conceal any gas cans. There are definately people out there that aren't above robbing fellow evacuees and it would pay to blend in with the crowds.

Something that comes up when it comes to storing gasoline is local laws and fire codes. At least some locals have a code prohibiting storing more than 15 gallons in a residence.