As long as the bottle pressure doesn't exceed the bottle's max psi, everything should be good. Then the valve should open up to relieve pressure. As for fires, my friend has been using a bottle heater in every single one of his cars for years and hasn't had a single problem with them. You could also get a bottle blanket, which is pretty much what I have. I put the blanket in the floorboard with the heater on high, let it warm up, then put it around the bottle.. the pressure goes up some, but not as much as what could be done with the bottle warmer.