I wouldn't think that driving around with tanks in the closed trunk of a car in Miami would be a great idea. It the gas pressure in the tank goes up the hotter the tank and subjecting the tank to those higher temps constantly does not seem like a good idea. I have never seen it happen, but I know down south people get pretty enthusiastic with filling tanks and an overfilled tank in the back of a hot car could put a damper on your day. Besides, how are your going to get a good fill on a tank if you need to wear oven mitts to get them out. Personally, in the summer I avoid leaving tanks in the car as much as possible and put a blanket over them to insulate them from the hot air of the trunk. Just the having the burst disk failing while driving would probably be pretty bad.
Even a big swing in temp, say from 80F to 140F would only net about a 10% pressure increase. That's really worst case as it ignores the thermal mass of a steel or aluminum tank. In other words you'd have to have a tank in a trunk for a loooong time for the air inside to reach 140F in a 140F trunk. I'd have to dig out the thermodynamics text books that I dont have any longer to figure that out.