A fire extinguisher is not a requirement for gas or glow powered engines. It's not a bad idea, though. Turbine engines have a much greater exhaust temperature, and any fuel spills making contact with a hot exhaust nozzle will almost always create a fire. If the fire occurs with the aircraft any distance away from the pilot, the aircraft will likely be lost, but spread of the fire can be prevented.

A gas powered plane can, and has been known to catch fire in a crash where the gas has been spilled and an ignition source was found, such as a shorted battery or servo lead. It's not common, but it happens.

The addition of oil to gasoline reduces the volitility of the gasoline somewhat. I would not want to depend on that reduction enough to drop a lit cigarrett into a can with a gas/oil mix.

I fly mostly gas models and do not have an extinguisher present when I do. I've thought about it a few times and admit to myself that I should get one and keep it near, but haven't done so yet. A little extra safety doesn't hurt, and it costs little.

In the meantime, standard levels of common sense dictate that you don't smoke or have or create an open ignition source around gas engines and the gas storage container. Keep the container in an area that is well ventilated and away from ignition sources. Don't carry a fuel container inside the passenger compartment of a car, pick up, or van. Storing a gas or glow model inside your home after it's been run isn't a good idea, either.

Think safe, act safe, be safe.