It can be all over.

Readability is my overriding goal.

Sometimes jazz composers write in C because it's too difficult to figure out what key is best overall when it modulates frequently. Just write with lots of accidentals in the notation.

Other composers change the key signature for the bridge etc. and back in the last verse.

But if I had to pick a key signature that would work best overall, I surely would not choose F# unless I had to.

Bob is right, sometimes the first chord of a song is the correct key signature. Unfortunately, this fails as often as it works.
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