I would say go to a regular vet first, cardiologists are great for cardiology, that's their focus, regular vets are experienced in general practice, and for a first check up, that would be the logical choice where the puppy can be examined for general health. If there's no reason to suspect any heart problem, there probably isn't any reason to go to a cardiologist.
MVD, the all too common heart disease of Cavaliers, including young adult cavaliers, evolves over time as the heart valve weakens and deteriorates, and is not the kind of thing that would be symptomatic in a young puppy. That would be unusual.
It's not uncommon for a murmur to be heard in a young puppy, but most often these are harmless and temporary (called "flow murmurs"), not a sign of heart disease. They disappear by 6 months usually.
Having said that, i wonder what age it's recommended to start heart testing on pet cavaliers, as a yearly screening test. Maybe someone can say something about that? Next year there will be a breed club sponsored heart clinic in my area and i'm planning to have Zack examined. He will be about a year and a half old then.
Perhaps it would be useful to take your puppy to be screened, but from the conversations I've followed on this forum, i haven't heard of people having their pets examined by a cardiologist as puppies, unless there is a questionable symptom.
The main one i remember is LauraD having taken Riley because he had a harmless flow murmur, and a cardiologist is best qualified to assess and diagnose murmurs and any heart symptoms. That was the first i had heard about this condition (harmless flow murmur).
PS I'm assuming you're getting a puppy. If you're getting an older cavalier, especially if you don't have much info on the health history, a cardiologist exam at this time could be a good idea, but i'd still suggest seeing a regular vet first, for general health.