In the video about 5th string bar cords Griffs talks about muting the 1st string on the C cord. The tab in the book only has an x over the 6th string. When Griff shows how to play the Cm and C7 he does not say anything about muting the 1st string. Likewise these cords only have an X over the 6th string. Can some one explain this to me? Why are we muting the first string on the C cord?