I am utterly left handed. When I bought my first muzzleloader it was with a left handed lock because I was was used to modern bolt actions where a dedicated LH action makes a real difference and I felt like I needed a LH gun.
Subsequently I have had the chance to shoot right handed flintlocks and for me at least, it makes no noticeable difference.
Remember that if you shoot a double gun you will have a lock on the "wrong" side whatever you do.
Stock geometry and correct LoP are much more important factors than which side of the gun the lock is on.
Also, if you do get a LH lock, remember that spare parts will be more expensive, harder to find, or both. My Traditions LH deer hunter just snapped its mainspring for the second time in two years, and yes I know Traditions are pretty much the worst you can buy, but even so, if it had been a RH lock I could have bought one on the high street and been able to hunt in Flintlock season. As it was I missed the season completely and am still waiting for a replacement from Traditions