Did you know that if you spay her before she goes into heat, there is almost a zero percent chance that she will get mammary cancer. If you wait til after her second heat, there is about a twenty-six percent chance of her getting mamary cancer. Also, if you wait until she is older to spay, you may have to deal with pyometra.
Of course there are a few risks to spaying but the benefits really do outweigh them for most dogs.
Breeding is a lot of work and it takes a lot of money to do health screenings and possibly getting an emergency c-section done, so just study all you can before you decide.
The youngest a Yorkie should be bred is second heat or 18 months and the first tme breeding shouldn't be done after three years.