Last evening I had a session with a friend who plays fiddle. I wanted to do a song in Bb, and asked if that was ok, she said "sure" She has always played in whatever key I needed to sing. If it is an instrumental, I'm willing to play along in any key. However, if I'm singing, it needs to be in a key that is right for my voice. Never paid much attention to that idea that this is the key it is "standard", or the one it is "Usually" played in. Did you know that Harpo Marx could only play in the key of A? Not very versatile.