In some societies people wouldn't advertise that they were left handed... we also get the word sinister from the latin for 'left' with all that implies. While no doubt there were numerous 'cag-handers' around, they would be trained to fight 'the proper way' (although left-handed fencers are supposedly a tad quicker than righties).
I'm old enough to have had a rap across the knuckles at school for using my knife and fork 'the wrong way'.
[/boring fact]
True. And being a left hander doesn't work in a shieldwall.
For adventurers, none of this would be a problem, though.
The main advantage I have found as a left-handed fencer is that you are used to fighting mainly right-handers, and right-handers are
also used to fighting mainly right-handers,so can't get their heads round the blade being on the "wrong" side. As the proportion of lefties increases, the advantage has reduced, unfortunately

Another boring fact: apparently one of the Border Reiving clans had so many lefties they built their spiral staircases going in the other direction, to put righties at a disadvantage!