I have burned the handheld flares in my backyard. It is interesting to see that after years of being "out-of-date" the handheld ones work just a good as a brand new one.
I tested the gun type flares during New Years and 4th of Julywhile shooting fireworks. From my tests, the gun flares do go bad (after a long time). I wouldn't keepgun flaresfor "backup" no matter what...when they go bad the flat out won't shoot!
If you do decide to burn up the hand flares make sure you aren't letting them "drip" into the grass or on concrete...the molten material that comes off is HOT and will catch your yard on fire or splash when it hits concrete!!
If you've never used a flare, I'd suggest "training" with your out dated ones.(DON"T SHOOT GUN FLARES UNLESS you wait until you are shooting fireworks! There are plenty of people who call 911 because they thought they saw a flare..which is good in my opinion!)