I believe you are right about storage being not being too risky, all the fires I know of have been on charge, in use or shortly after use.
I store mine in the fridge, low temps (but above freezing so there is no crystallisation of the electrolyte) slow down the chemical reactions, and minimise the capacity loss over time that is always occurring. It also means that I see them several times a day so if there was an abnormality in a pack I could isolate it and dispose of it minimising the risk of fire. Should I have a fire it also doesn't seem to be a bad place to have one, in a contained cold box to neutralize some of the heat, but with a huge pressure relief door so there isn't an explosion, in a room with a tiled floor, no soft furnishings, a smoke alarm, 2 different forms of fire extinguisher, and a water source.
If you use an ammo can to store LiPos don't forget to drill some holes in the lid and remove the seal for pressure relief, or you at turning a potential fire into a potential explosion.
Storage in -20 will chrystalise the electrolyte severely reducing your LiPos capacity very quickly, they would be fit only for the bin after months of it. Storage at 90 is better but will still reduce their lifespan, and if the container gets direct sun on it it could get much hotter inside, I once puffed a lot of my LiPos by leaving them in my car for a weekend in 90 heat, it was much hotter than 90 inside, maybe it went over the 140 that can kill a LiPo in seconds, or maybe 120 sustained for hours will do it too.
Paul Upton-Taylor, Greased Weasel Racing.
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