Yes, if you are concerned then putting the LiPos in your pouch in a steel box to reduce the effect of a fire is useful.
Let's be clear - a LiPo can go up at any time irrespective of its state. Charge, discharge, storage. Research papers conclude that the major reason for this is faults during manufacturing that cannot be eliminated because of the many variables in use of chemicals during the manufacture of this type of battery. Another reason is the growth of things called dendrites that occurs during use of a LiPo, and more likely during abuse of a LiPo. Overcharge of a lithium-ion cell can cause significant degradation of both anode and cathode. On the anode, overcharge can cause plating. Plated lithium forms dendrites (hard crystals) that can grow over time and then cause internal shorting.
The plastics separating the anode and cathode are about 0,030mm thick, or about 0.001" - 1/3 the thickness of a human hair. Anything that penetrates these separators causes an internal short. Sometimes that short melts the plastic and seals it off. That is usually accompanied by some puffing as gasses are released. Sometimes the puffing is a prelude to a short.
When a cell is on charge or discharge, the cell has a source of energy. If something happens that causes a short then during charge or discharge, the charger provides additional energy to the problem. The charger feeds the short, and the discharger keeps taking energy out so also feeding the short. These are the most likely times for a problem. The least likely time is during storage at the recommended storage voltage.
Do not be scared of LiPos. Problems have arisen when chargers are incorrectly set, or people deliberately set out to ignore instructions and advice by charging above the 4.2v/cell limit, or discharging below about 3.2v/cell. This is to be expected - ignore advice and you increase risk.
Use a sack properly - push the cell to the bottom of the sack, fully fold and seal the lid and avoid the 'boxes' as they have too many gaps for the fire to get out. Don't overcharge, don't over-discharge and store between 5 and 25 degC. If you feel better about having your stored LiPos in a sack in a metal tin then that's good planning. HTH
