to cover a few points, your gas cans should have a proper seal and not vent any fumes or gas but as said, they will expand due to altitude/temps and normal off gassing. Over time the cans pressure increases too much and if the gas can cannot expand, the pressure will have to go somewhere, normally past the gasket and out the lid. It's a good practice to open your fuel cans daily to help vent any excess pressure to prevent leaks, having the spout upright makes this much easier so you don't need to undo straps that are holding down the cans.
I have noticed my rotopax will bulge slightly when filled due to the pressure building, but i have never had an issue of the rotopax seeping or leaking even without venting them daily. Clints gas can mount is pretty snug with 5 rotopax and i think that may have been why his seeped a little bit on the DV trip, they may not have enough room for expansion. This is not a huge issue as they are outside the vehicle and it just collects a little dust. Now if they were mounted in the cabin, it would be a different story, like Randy's adventure out on the Mojave. Me personally, gasoline belongs outside the cabin, the chance of a spill and having to deal with the fumes is enough for me to not want to chance it. Kudos to those that have had no issues. I see no issue with having jerry cans on the roof laid down, as long as you have a good gasket(I would carry spares), if you are not sure about the seal, I would only fill the can with 4 gallons to allow enough room for expansion and keep an eye on it, stand it up if it starts to leak.
In CA, thanks to CARB, its required that gasoline containers be red, diesel is yellow, and kerosene is blue. All of which are the standard colors associated with each fuel type.
Containers also need to have identification for the type of fuel,
"embossed language, or permanent durable label “GASOLINE” in minimum 34-point
Arial font or a font of equivalent proportions"
Reference:
http://www.arb.ca.gov/regact/pfc/2005/cp501.pdfI have never heard of any problems but i suppose someone having a bad day could cite you or take your gas MadMax style.