Propane is a great refrigerant. It's used in industrial applications because of its high efficiency. And I wouldn't let people give you too much crap.

You're driving around in a machine filled with 20+ gallons of one of the most highly flammable substances in existence (gasoline), another 3 or 4 gallons of highly flammable oils and lubricants of various types, along with flammable and toxic plastics and several gallons of 200+ degree scalding antifreeze.

Compared to the ordinary risks involved with just operating a car to begin with, the 1-2 ounces of propane in the a/c system aren't really going to change the equation. I wouldn't worry too much.