I have a few NATO cans here too. They do seal very well, Which can make things interesting.
If I fill up the can when it cool out and then try to open it when its hot out, I have to use pry it open. So far, I haven't had gas spew out, but It is a possibility. Keep in mind, we can get 30F+ low to high temp swings in addition to the daily/seasonal swings.
I usually try to rotate the fuel at least every 6 months since we can't get anything except e-10 around here and that stuff doesn't keep well.
~Mark