The idea that fiddlers traditionally play in certain keys may be true, but what keys those are depends entirely upon what tradition you're talking about. Bluegrass fiddlers routinely play in B natural. They may all prefer to play in G, but when Mr. Monroe decided he liked singing in B it was "learn it or leave the band". On the other hand, a country or blues fiddler may spend a lot if time playing in E major while a an old-time fiddler can live his entire life without playing an E major tune. And I've played with Cape Breton style fiddlers whose medleys have certainly included tunes in Bb and F. It's all in what you get used to.