[Darner]

I've read that ambering or yellowing is the first indication that the nitrate base is breaking down. There might also show some mirroring in the silver emulsion.

I agree that the way sheet film is usually stored does make it safer (smaller quantities, individual envelopes, not in air tight containers, room temperature) but it's the same material and still has to be carefully handled. For conservation sake I'd put them in cold storage to prevent deterioration and I'd keep them separate from other kinds of negatives in case they do give off any gases that can damage something else.

Probably the worst case scenario is someone stores a bunch of nitrate negatives closed up in a box and they decompose over time in a hot attic.