The question of appropriate keys - particularly at sessions - can be a little thorny sometimes. One local example that springs to mind is the tune "The Marquis of Lorne". According to one tune book I have, this is normally played in G. However, at a local, regular session, it's played in D - simply because the bloke who leads us into it finds it easier to play it in D on his Northumbrian small pipes. So - even when he's not there, we play it in D! I play it at home on my tenor banjo in G because the fingering's easier. If I play it at the session, then I use a mandolin, where the fingering's easier in D. The point is that no key is absolute - it can depend on the environment and circumstance in which you play the tune.