Your thread title gave me flashbacks to the battery safety program I used to be involved with at work. :dread:
If you're concerned with the risk of thermal runaway and you're not storing the camera for a long time, I'd leave it in the camera. Thermal runaway occurs when the battery gets shorted out and usually results in flames. The least chance of an accidental short is installed in the camera, although the EN EL-15 in my camera seems to be pretty well designed as far as keeping the contacts protected. Most of the catastrophic failures of batteries installed in devices that I've heard of are the no-name/off-brand/counterfeit batteries. The name brand ones are pretty reliable (and when there's a chance they aren't, they're quickly recalled) as long as they're not shorted out.
Last time I checked, in the US the TSA will only let you put Lithium batteries in checked baggage if they're installed in a device or sealed in their original packaging, so that tells you something about what they think the safest place for them are (although, you could also probably fill a page with things the TSA that don't make any sense at all). Lithium batteries were also being investigated as a possible cause for the fire that caused a UPS jet to crash in Dubai a couple of years ago. It's just one of those trade-offs that comes with having a battery that holds enough energy to last for a long time.
Mine stays in my camera unless I'm charging it, but I usually find myself using my camera every few days as well.
Hope this helps!