I am not a breeder, and I'm far from being a breeding expert, but I want to second what Connie (missjennygirl) has said. As she noted, we attended the Tom Coen Sheltie seminar yesterday. It was attended by a number of knowledgable people who've been breeding for a long time. But it was also attended by performance and agility people, who wanted to learn more about how the proper structure of a dog makes it perform better. Tom began by showing photos of the very first Sheltie...a dog no one would recognize as today's specimen! As the program continued, we saw in photos the evolution of the breed, dog by dog.
The most amazing thing to me was that at the end of the day, Tom proved that we can trace our individual dogs to the early Shelties by knowing their breeding. That is what lineage is, and responsible breeding produces. I admit some of the material was way over my uneducated head, but I came away with a knowledge of the breed and its structure that I didn't have before, which illustrated how important it is not to breed dogs who do not conform to what Shelties should be.
So it is important to do your homework on a breeder before you buy. Know where your dog is coming from, and who its sire and dam are, and who their parents were. Read one of the many books about Shelties and their origin. It's fascinating and educational. We'll all be better dog owners for it, with healthier and stronger dogs.