Apart from being right or left-handed, is the relative development of the bass hand and melody hand not also a case of playing style?
In France, where cross-rowing is the norm, then right from the start players learn to find fingerings which will suit the direction of the basses, in other words the tunes are chord-led.
In Ireland, they often don't worry too much about the basses at all, but concentrate on getting the style and ornamentation of the melody.
In England it's somewhere in between, and of course one-row players are somewhat limited anyway!
That's so true. I came to the melodeon sort of by accident / curiosity many years ago, and, apart from listening to a lot of folk music, I was not part of any particular folk tradition eg. knew nothing about morris dancing, tune history, folk clubs etc. I just learnt this bizarre instrument as it presented itself to me! I never even thought, for example, of playing a tune just sticking to one-row, and the first style I actively tried to imitate was the european style - cross-rowing and bass runs, with bass chords guiding treble buton choices. So the left hand has always been as important as the right in my playing - it's got nothing to do with being left or right handed, I think (I'm left handed).
However, maybe being left-handed give you slightly more dexterity (or sinisterity) with the bellows and air button use...