I know this can be worrying. Unfortunately there is a very high rate of incidence of this condition in the breed just as there is a very high rate of incidence of early onset heart problems and when we choose this breed, it is sadly a concern that comes with cavalier ownership and never goes away. That's why I feel it is so important to work with breeders who acknowledge these problems and work actively towards bettering the situation for the breed through health testing, supporting research, and pushing their breed clubs to keep these issues as top health priorities.
All dogs scratch, and they scratch for many reasons, from a plain old itch to fleas, skin conditions, ear infections, ear mites, PSOM (glue ear), allergies. IN other words there are a lot of reasons that are more likely for scratching. If the scratching isn't of a type that is generally linked to SM, then I really wouldn't worry.
The chances that any cavalier has SM is fairly high over a lifetime, going by research samples in several countries (somewhere between 40-70-ish% depending on the age of the sample). So, as with MVD, the issue should probably not be worrying about whether a dog will get the condition, but to assume it probably will eventually get it, or at the least is going to have the skull malformation whih can at times cause problems on its own and cause syrinxes to form at any age. But realise most dogs go a lifetime without showing symptoms or else showing only very mild symptoms. As with MVD the age and rate of onset is enormous but what is usually of most concern are obvious symptoms at an earlier age (eg under 3.5).
What you are describing aren't really obvious symptoms -- so again I wouldn't get worried about this unless you are seeing something that is clearly wrong and odd. If you are really worried, you can go see a neurologist who can do a clinical exam and tell you if there are signs of limb wekaness or pain in the spine or limbs or neck -- they can check for this far more accurately than a vet.
In other words if you are not seeing something you feel you need medication for, then I would just try to stop worrying. Just remain aware of what to watch for and if you are uncertain watch the videos I have on the SM site as these give a pretty good indication of certain types of excessive scratching and (especially in Chester) other symptoms.
In the same way I watch for signs of MVD.
If anything most of us have more to worry about with MVD in terms of a threat to our own dogs' lives.
I live with two dogs with SM (Lily is most likely affected as well) and they lead quite normal lives, albeit on some medications. It isn't the option I would have chosen for my dogs of course but so far has been very manageable and doesn't overly compromise their lives. There are dogs at the other end of the spectrum as well -- as you can see from the videos here:
http://sm.cavaliertalk.com/video/videos.html
And of course I never know where mine will be a year from now. But as a person with dogs living with this condition, if you find yourself in this situation, you just deal with it. There are numerous approaches that can be taken. But most important is to keep pressure on the clubs to take this condition seriously, to support research as best you can, to ask breeders what their own stance is before you buy puppies, and consider whether you wish to opt for puppies from scanned parents. There are many options in the direct hands of the puppy buyer and every cavalier lover to work to improve the current situation and make sure it doesn't get worse.