In his book "Rock n Blues" Jon talks about notes that can be played anywhere in the chord progression and wont be wrong.

Been listening to Dr. Ross in an old recording of a song called "juke box boogie",
I was concentrating on the chord progressions that the band was playing.
Sure enough they were playing I I I I, IV IV, I I, V, IV, I I.

On the harp, the Dr. stayed on the I chord all the way to the V chord. He was working the 2 draw (and variations) the full eight bars. The band changed to the IV and he did not. Still sounded great.