Doesn't it really just come down to whether you are in a sharp key or a flat key? I can't think of one "flat" key that has a sharp in it ANYWHERE in notation. So if you are in F, Bb, Eb, Ab, Db or Gb, you'd use Cb rather than B# because sharps don't belong in a flat key. BUT, it's also a B, so...And there's the rub. Nobody that knows notation would write an Ab minor using Ab, B and Eb. They'd use Ab, Cb and Eb. But if you are in a SHARP key, G, D, A, E, B AND F#, if you should need that note, it's be a C.

Is that right Matt?