[vanderZwan]

Doesn't left-handedness give an advantage in most sword fighting and fencing sports? As demonstrated by the percentage of left-handed players at the top level is higher than the population average? I heard the same thing is true with boxing, presumably other martial arts, and tennis (and who knows what else).

The explanation I've heard is that in any sport, on average one would train more with right-handed opponents. So when facing a left-handed opponent you are significantly less experienced, which results in left-handedness having a slight advantage.

That is a bit different than what you describe (since you're not allowed to use your katana in a left-handed fashion), but still.

[chupasaurus]

Can talk about foil fencing perspective: lefties get a competitive advantage only in junior years because training exercises are done against same-handed opponent (be it a coach or dummy) while they spar and compete mostly against righties. It diminishes with time.