Shato, I too have heard of what you explain, but falcon beat me to my answer, but with more detail than i knew of. I have heard more than a few reports about tests of ammo being exposed to high heat and even open flames. It was found to be as falcon reports....no big explosions from ammo being heated or burned. Discharges yes, but no violent explosions.

Now of course I don't want any of my ammo exposed to undue temps, but the ammo cans were created for a reason (every military power has used some form of ammo cans) and they have a strong seal which impedes the collection of moisture.
I'll continue to store ammo in ammo cans until I find some imperical evidence that says it's not good. For now, my own experience is that my ammo all works fine, and the fire thing has been dismissed by a # of studies.
Not sure I want to take my all my cues from the BATF folks anyway.
I don't store my powder in such containers, just my ammo.