I think these links can help you answer you question. It's important for you to understand, "why" you want to breed. Knowing that answer, you will better be able to come up with the right answer for you. I think that only those breeders who have a burning desire to breed, should breed. They are fascinated with the subject and want to read everything they can on it. They aren't doing it for money because in the beginning, there is much more money put out than taken in, especially if you do the proper health screening. So many people have bred Yorkies that have genetic health problems, and unfortunately many of these don't show up till after the age of 3, so you could be passing along serious illnesses if you don't test for these things. Most breeders don't want to shell out the extra money, and vets don't always recommend the health screening because they aren't experts in the breed, and don't understand all the health concerns associated with yorkies. I personally think it's irresponsible, to bring a life into the world that your not 100% sure has a good chance of being free from genetic disease. Health screening and studying the lines of the breeding pair will help with this, but even then there's a chance that you could pass on a genetic trait and this means that you should be prepared to take back the puppy and make sure it gets proper health care.
Breeding Your Dog Dog Owner's Guide: Ten reasons to breed your dog Yorkshire Terrier