I've always found that the most important thing is to the learn all the notes of the buttons. That will give you the ultimate freedom of just knowing where stuff is at. Put a little music theory (very basic) on top of that, and I think anyone can play in any key. Obviously, with increasing limitations depending the key being played in relative to the base key of the accordion (some key-to-accordion match ups may limit you to only 1 or 2 notes, and maybe not even the root note of the tune's key). Considering some of these extreme limitations, some would argue that it isn't even worth playing if the key-to-accordion match up isn't in the norm, but if you know what notes the buttons are you at least don't have to put your accordion down when someone switches things up on you.