GreyDawg, I wish I was staring at the Grand Tetons pondering the safety of my butane containers.
I have one of the Butane stoves that you see in Asian markets. With no propane on board the van I'm committed to use the spray paint can type Butane canisters. On my stove model I leave the can that's in use in the stove but release the lever that compresses the can to feed fuel when I'm not using the stove. I hope that's an understandable description. Not sure if the REI dual fuel is designed like that.
Here is some info on butane vs Propane so you can decide what best practices you are the most comfortable with.
Butane has a much smaller range of vapor pressure based on temperature than propane.
Here is a graph
Assuming the fuels we are using are 100% butane or Propane (less impurities) at 120 degrees F butane is only 47 PSI vs propane at 200 PSI. This is why the 1 lb green propane cans are relatively heavy duty and the Butane cans are like spray paint cans. The Butane cans are designed to vent in an over pressure condition. The top crimp of the can will split slightly and vent out the contents at a set pressure. This method is called a Rim Vent Release or RVR. The RVR in butane cans are supposed to vent at 200 PSI with visible and audible deformation of the top as a warning before venting. So armed with that knowledge even if you placed your butane can on the dash of the van, in the sun, in Phoenix and your butane got up to 175 degrees you would be at 132 PSI. Theoreticly well under the 200PSI vent pressure. I don't recommend doing that but the idea keeps me from worrying about temperature in the cabinet in the van.
Fun Fact - Butane below 33*F will not vaporize and you can carry it around in a bucket. Which is why many camp stoves mix other gases, acedyline, propane, ect to improve the cold temperature performance. Which is why all the above info is out the window with mixed gasses.
Hope this helps put it in perspective. I'm more concerned with my butane cans not leaking out the valve when off, keeping the little red caps on unused cans so they don't get pressed accidentally, or just rolling around.