The preferred position depends on the scale of the tune you're playing.

Some tunes use a relatively simple scale like 5-notes or penta-(=5)tonic, and would be playable in both positions.

A fiddle tune like Arkinsaw Traveller, Melville Twostep, would only be playable in C on a C accordion because of the notes used within normal major scale.

Some tunes like Old Joe Clarke, Acadian Twostep, Cajun Hot Shoes, use a mixolydean scale, that is a Bb instead of a B in the major C scale.

see: http://en.wikipedia.org/wiki/Mixolydian_mode

You'll need an F accordion for that to play it in C as such, second position. Or play it in G on a C-accordion.

Some tunes are somehow playable on a onerow melodeon skipping an occasional less important melody because you only have a limited scale on it: 7 notes available per octave instead of the 12 on a "less abnormal" instrument like a fiddle, guitar or piano accordion. - Nout

I don't have any examples, Bryan. I wish I'd taken more notice at the time the post was put on. Maybe I read it wrong, but it just stuck in my mind.

Interesting theory, all the same.

BJ