WARNING: Lots of things mentioned below are relatively dangerous practices that I wouldn't advise now and I am glad I didn't learn a harder lesson. Impressionable young minds beware.
Well, it turns out I've done some experimentation on this issue myself in my younger/bolder/not so smart days.
I was sitting over at a friend's house drinking beer and cleaning guns after a day of shooting. He asked me to pass a box of ammo that was sitting on the coffee table and as I did, I moved it over the lit candle on the table. He reprimanded me for this as he felt there was a danger of the round cooking off from this. I maintained that at no time had I placed us in any danger. I stated my belief that the powder ignited at way too high a temperature for us to be at risk from a mere flame.
Being the carefree rocket scientists we were, we decided to put this to the test by pulling the bullet from a .22 and dumping the propellant into the candle to see what would happen - no safety goggles, no thoughts about dumping propellant into a fire on the coffee table... just an intense scientific curiousity
Well, we dump the propellant into the candle and.... nothing. Absolutely nothing happens. Both of us being kind of entertained by rapid combustion, we were a bit disappointed. We expanded our hypothesis a bit... obviously the powder burns at a really high temperature and is stable; but what about the primers? Could passing them over a candle represent some danger?
We took the empty .22 casing and some needle nose pliers and throwing caution to the wind, stuck the casing over the candle flame (the same one we had just dumped all the propellant in). Well, after about 2 minutes we were both surprised and frustrated and how difficult it was to make these things go... BANG! The primer finally detonated - not sure whether it made the powder go or not; but it blew candle and hot wax onto the ceiling and all over the living room (as well as the coffee table and assorted frou-frou stuff).
From upstairs we hear his wife asking "<name deleted for posterity's sake>, what was that?" to which my quick thinking friend replied in his best soothing voice "Nothing honey!"
All eyes and ears accounted for, brass case intact in this case; however the mere proof that I am still alive and relatively uninjured should not be taken as evidence that all of my methodology is safe or even sound