It might be your idea of common sense but putting a can of flammable liquid on the exterior of your vehicle can be just as dangerous and far more likely to be stolen, or punctured in an accident. I've seen 2 trucks roll and dump gas all over from Jerry cans on roof racks- and one M151 that was rear ended and dumped 5 gallons of gasoline all over , That leaves what? the front bumper? Also remember don't assume all jerry cans contain gasoline, just as many carry water, diesel, or motor oils.
If it is secure and not leaking fumes it is safe inside.