Rory has SM and he definitely scratches w/ intention. I just want to point out that not all SM dogs do the "air scratching" or do it seemingly absentmindedly. He almost always makes contact w/ skin (and has little scabies to prove it) and always seems to do it intentionally. (i.e. stops, sits down, scratches furiously). And there doesn't seem to be any 1 side he scratches more or less. He scratches head, neck, ears, top of head... Sometimes bits his back legs and knees.
He also has PSOM in one ear, and although this could be one reason for some Cavalier's deafness, it isn't the only reason Cavs may be predisposed to deafness. The white spotting in blenheims and tris can be the cause of deafness in any breed with white spots. The cells that cause the color also need to be present in the middle ear for hearing to function. So dogs w/ white spots are more likely to not have these cells in the middle ear and then be deaf. Same cause as Dalmatians.
But yes - there are LOOOOOTS of reasons for scratching, not just SM! So be careful not to over analyze. Ask others who live w/ the dog if they think he's scratching more. And know it's very uncommon to be seen in very young dogs (underr 5 or 6 months). I know it's easy to become obsessed w/ worrying and freaking out every time your dog scratches. Just remember that ALL dogs scratch! It's OK! Do what you can to determine if it's SM and try not to stress out too much.
Dogs are not our whole life, but they make our lives whole.