BY all means pratice with a few...but outdated flares are still perfectly legal and sensible to carry onboard as spares. I've seen 'em ten+ years past date still work perfectly well.
The big surprise is that if you've never fired one...unless you're firing SOLAS flares or something you got at Missles-R-Us, you'll be amazed at how poorly marine flares compare to plain civilian fireworks.
Put the old ones in an airtight jar or tin, with moisture and air excluded they kept just about forever. Those plastic Olin containers? Are designed to make them go stale.