Just to add a little confusion (though hopefully not too much), I have to disagree with Andy's comments that the 12->5v converters are likely to be linear. The power dissipation (9W for a 1A converter working from 13.8V) is just too high for the cigarette lighter plug packs. They'll almost certainly be switchmode devices, running in the megahertz range. These high frequencies allow the inductor and caps to be very small, and they easily run at >90% efficiency so they're dissipating less than 500mW (and they're still as cheap as chips).
Their safety basically comes down to the components used, and to put a rough rule of thumb on it, the more expensive, the better. Why? Two factors. The first is that the switching FET - one of the more expensive parts @~20-30 cents) is subject to all the spikes that exist on the main bus, and in any automotive or aircraft system, they can be very large indeed. If the FET fails, it typically fails short-circuit, not open. That then puts the full 12-15V of the bus onto the 5V input on the electronic equipment. Most of the time, it'll blow a fuse (either in the equipment or on the panel), but if you combine it with the large spikes that took out the FET, they could take out the battery too (with extreme prejudice). Given that these spikes tend to occur at engine start and shutdown, it's wise to switch off the power to the converter before starting or stopping the engine (just like you always do to protect the rest of the avionics, don't you!).
The second factor (though it doesn't affect the battery safely) is RF conducted & radiated emissions. It's not hard to suppress the emissions from a power supply if you do it at design time, but if you're either slap-dash in your design or looking to shave every cent you can from the BOM cost, then it's easy to 'accidentally' leave off the components that make the unit meet the international specs. It is illegal to sell electrical equipment in Australia that doesn't meet the requirements, but if you're buying cheap (and especially if you're buying off the web), there's every chance it will radiate like crazy. There's no easy way to check this your self, but basically, if you use a converter and you start hearing noise in the intercom or radio, chuck it and go and get a better one. Life's too short to stuff around with making a $5 converter work in your system.
Bottom line? If you're powering your $800 iPad, don't use a $2 converter.