Fuel cans in general are dangerous things. The emptier they are, the more dangerous they are 'cause it's the vapors that can cause the problems. I agree that, regardless of the type of fuel can, you should use great care to avoid sparks of all types.

There are a lot of cases documented where Joe Suburbanite throws a plastic gas jug in the back of his pickup to go get gas for the lawn-mower. Between the residual gas sloshing in the plastic jug, and the plastic gas jug sliding around in the plastic bed liner, there's an enormous static charge built up by the time he gets to the corner gas station. I have felt it pull the hairs on the back of my hand. When he touches the (grounded) gas nozzle to the jug to fill it, he gets a really ugly surprise, new eyebrows,and sometimes a new truck. Many gas stations have signs posted telling the customers not to fill gas cans in the bed of the truck because of this. When you remove the gas container from the truck and set it down on the concrete, it lets the charge bleed off.

With a metal can, charge is easily dissipated. That's why you see the fuel attendant attach a ground wire to full-size airplanes before they are fueled. THe fuel truck also gets grounded, so EVERYTHING is referenced to the same potential and there are no sparks. I fill mine through the fuel pump anyway. (it's tedious, but at least it takes a long time. An electric pump would be an advantage here.). I think it would be a bad idea to fill the lay-down type can through the cap, because removing the cap would cause wear and potentially leakage.

If there is a spark to a closed metal can, it will be conducted through the metal and not affect the contents. This is why they tell you a car is a safe place to be in a thunderstorm. If lightning hits the car (metal can), it just goes around our little pink bodies harmlessly. It's not because of the rubber tires. I don't know how some people formed the opinion that a lightning bolt that has just jumped two miles would be stopped by four inches of wet, carbon-filled rubber (with steel belts).