The cars power source, the battery and alternator, fluctuates, both above and below 12 volts, that is why using the 12volts from the car to power a 12 volt device, is a bad idea, no matter where you take it from.
The major issue with the Cig Lighter is the lack of AMPERAGE/WATTAGE available. Most cig lighter sockets are speced to about 5 or 7 amps, or 60 to 75 watts. Drawing more from there is a bad idea, since the wires that go from the battery to the cig lighter, are thin and will most likely burn, and cause your car to be a hulking piece of red molten metal.
Supplyinh less then the needed power to your devices, while not "lethal" to the equipment, it will most likely cause it to act erratically. You will have wierd issues that will be perplexing, like reboots, and read and write errors, and just strange operations.
DO it right, don't skimp, it will save you time and headaches.