Dunno about fiddle, but as a general rule, ... flat keys [are] easier to play on woodwinds and brass. This isn't a matter of mind-set, it's a matter of fingering.

It is true it is a matter of fingering, but the easiest keys to play in for woodwind, at least, tend to be those closest to written C. Many woodwind instruments are transposing instruments and the written key and the sounding key are different. The reason flat keys are often preferred is because a sounding flat key is written in a key closer to C. An obvious example is the Bb clarinet where playing a written scale of C major gives a sounding scale of Bb major.

I play recorder and the easiest key to play in, on a C recorder is G, followed by D and C. As you move away from these keys, either to more sharps or more flats you have to make use of cross fingerings which can be awkward.

Cross fingerings also affect the quality of the sound which may be a partial explanation of why different keys have different character - even if you are playing nominally in equal temperament.