The thoroughbred racing world has an old adage that says; “breed the best to the best and hope for the best”. I think this would hold true in any breed. The main goal is to pass on the best mental, physical and conformational traits of that breed, without using horses that will pass on mental, physical, and conformational weaknesses. People would refer to this as “weakening the breed”
One must take in all the important factors before breeding including age, conformation and certain “faults” that might not want to be passed on. Also, it is important to consider “breeding soundness” of both mare and stallion and to research certain “nicks” or patterns that have been successful in certain breeds.