Since I am new new to the accordion, I play most of the banjo, fiddle, and guitar tunes I know in C. This helps me learn the fingering. I also try to play the A and G key tunes I know in G and can usually get by if I play an A note instead of the F natural.
Unfortunatly this does not work if I play with the group because they play mostly in the fiddle keys of D or A.
My plan is to get good enough and then buy a D box for playing in D and A. At my current rate I will have plenty of time to save up the money.