Animal Reproduction. American College of Theriogenologists
This summarizes my opinion on telling somebody that they absolutely should alter their animals. Thankfully osteosarcoma and hemangiosarcoma are not major concerns in Yorkies, so the decision is a bit easier.
Intact and altered animals do have different caloric needs. Just feed accordingly. And behavior/personality changes are absolutely possible because it is influenced by hormones.
I choose to spay for health reasons, but not automatically before the first heat. I choose to castrate for behavioral and safety reasons, but again not at four or five months just bc somebody says to.
My girl went into heat several times. It does not make much of a mess with the small breeds. Doggie diapers work if you don't want red dots all over your blankets.