First: there's a lot of misinformation about puppies and SM. I know of definite cases where 7-8 week old puppies have shown symptoms. It is just more common for it to present after age 6 months or older -- mostly because it is progressive -- but it does show up in dogs much younger than that. Dogs that are showing symptoms younger than about 2.5 years old tend to be more severely affected. For that reason it is very important, IF you are seeing ANY single, or combination of symptoms that could be SM, AND a vet can find no other specific reason for the symptoms, to ask for a referral to a neurologist. Only a neurologist can confirm whether it's SM. Sometimes this can be done on strong suspicion from clinical signs but generally diagnosis requires an MRI. Vets will be unlikely to consider the symptoms as SM which is extremely rare in other breeds. In their entire career they would probably never see a dog with SM.
Generally the scratching ** when related to SM** is a sign of some pain and discomfort -- Dr Rusbridge believes this is a sign of damage to the dorsal horn of the brain and is a reaction to consequent pain. So don;t underestimate scratching as a symptom -- it is a significant one if it is there. My Sm dog Leo has never had any other symptom except scratching but it has become severe enough that he is on a neurological painkiller to control the scratching fits. I don;t say this to scare people but to indicate that IF no other cause can be found, please don't let a dog potentially experience the discomfort of this condition on the assumption that there isn't any pain if there's 'only' scratching.
On the other hand it is important to consider if and why you need an MRI. If you want to try some of the medical approaches first you can follow Clare Rusbridge's protocol with your neurologist and vet's approval; this is downloadable from my SM site, www.sm.cavaliertalk.com. If you are not going to consider the possibility of surgery, and you want to treat without knowing exactly what the dog has (extent of syrinxes etc -- which are things that may influence a decision for surgery) then I wouldn't get an MRI unless covered by insurance or you can vaail of a low cost scan somewhere.
For anyone who suspects anything in what they are seeing: try to capture the behaviour on video and also keep a diary of every time you notice what you think is a symptom. Trembling legs are a concern as that does indicate the dog can't fully support itself for some reason. Some dogs do shift from foot to foot when sitting (jaspar does) and you might see some leg trembling but really if this is consistent I'd want to have it checked if it continued.
With Leo, I did not keep a diary, and his scratching got worse so gradually that I didn't notice just how bad it was, and how abnormal, until 1) I filmed it, the video that is not on the SM website; and 2) while grooming him I noticed he'd torn large chunks of hair from his ears from scratching. I already knew he had SM as he was MRId for ersearch when a year old, and I should have been watching more carefully, but really, you get so used to your own dog's behaviour that you can fail to see worsening unless you have something to gauge behaviour by.
With puppies I'd be inclined to not overly worry about miscellaneous small things and wait and see. Also have a look at Leo's video -- this is SM scratching, not normal scratching. All dogs scratch. Puppies often have mites or fleas. Leo didn't scratch for a year after he was diagnosed with SM.