Dunno about fiddle, but as a general rule, sharp keys are easier to play on stringed instruments and flat keys easier to play on woodwinds and brass. This isn't a matter of mind-set, it's a matter of fingering. For example, with the guitar, the easiest keys to play in are C (no sharps or flats), G (1#), D (2#), A (3#), and E (4#). Going the other way on the circle of fifths, in F (1b) you're already into barre chords, and the more flats you add, the more barre chords you have to play.

'Course a capo solves all that.

One of the most difficult pieces in the classic guitar repertoire is Fernando Sor's Etude in Bb (2 flats--in the folio of 20 Sor studies that Segovia worked out the fingering for). Barre chords all the way. Beautiful, but a bitch to play. (No, I can't play it. I get about six measures into the thing and my fingers fall off!).

Don Firth