I'm of the mind that any flammability risks from hydrocarbon refrigerants aren't worth worrying about. I'll point out there is no oxygen at all inside a properly serviced a/c system - hence no possibility of an explosion. If the system suddenly dumps its charge then yes. But lets consider that your car is already carrying 150lbs or more of fuel. In a modern gasoline car this fuel is pressurized to 35PSI or more at the tank and pumped to the front of the car. The engine is also filled with flammable engine oil, the brake system with flammable fluid, and if you drive an MB with self leveling...well, you get the idea.
That said, I'm not using them. They are illegal. My stash of R-12 is gone. Since my 124 wagon upchucked its compressor last fall, I've decided to try R-406a. It supposedly cools better than R-12 - a good thing if you're driving a 124 wagon in Texas. Will let you know how it goes after I get around to charging the system, and after if heats up a bit down here....