Unless there's a LOT of inter-song transposition and some pretty whacked out chords, I am a big believer in the number system. Instead of learning C-F-G-Am, you learn what they are relationally, and then learn the song 1-4-5-6m, and now you can play it in any key you want...

It's a big part of Nashville sessions, and helps you quickly understand the relationship between chords, and once you understand the relationship, you start to understand the 'WHY' of a chord sequence. Once you understand the 'WHY', you are beginning to get to the core of what music is about....