Originally posted by Mikka:
To get back to the original question, whilst it is obviously useful to be able to play in all keys, one compromise is to concentrate only on what are known as the "busker's keys", that is to say C, Eb, F, G and Bb. This puts you within a semitone of any given key.
Mikka, that's really good advice and covers most of the jazz standards, but if you venture in to pop music, rock, folk, blues, etc., you'll need to be comfortable in the keys of E and A, as many guitarists write and play tunes in keys built around open string chords: E, A, D, G.
As for blues, even when the play in "A" they're really talking the A blues scale, which is closer to A-minor, ie, C major, so if you stay mainly on the white keys when playing A blues with the band, you can stay out of trouble.
Now that I'm thinking about this, I'm realizing that it is difficult to make "rules of thumb" for playing tunes that cover 40 years or more of musical styles. Take, for instance, the bossa novas of the 1960s, that are fun to play and very popular with the audience. They can be very difficult for a student to analyze harmonically because they're often built on cadences and root movement instead of the familiar diatonic progressions found in most jazz standards.
Perhaps this is why these tunes sound so fresh--even today. Their movement is unexpected harmonically, but has a logic in their pedal or melody, which can take the tune through seemingly unrelated keys with each change.
I'm thinking of Jobim tunes like Ipanema, Corcavado, and Meditation, but there are others, like Cole Porter's What Is This Thing Called Love, that's been around a long time. It's usually the A parts of the tunes that work this way, with the B parts using a more familiar diatonic progression.
Playing the A part of tunes like these requires more chromaticism than playing the B parts, and the inexperienced player may find herself in unfamiliar territory from one measure to the next when comping or improvising in these sections.
So I guess the only answer is: learn to play in all keys.
By the way, there's way more than 12 "keys", as we're really talking about "scales" when we mean "keys."
But that's another topic.
