I had always heard of the risk of urinary incontinence if pups are spayed/neutered too early, and I also wanted to wait with a Yorkie to see if their baby teeth fell out and had any baby teeth needing to be pulled at the same time while under anesthesia. From reading on YT and talking extensively with a dear friend about this, I now would prefer to wait until after the growth plates would be closed, which would be after the first heat.
I did read on this breeder's website that not all pups are spayed/neutered before they leave. They look like wonderful breeders, and I think it's very sad that, because of people who don't abide by their agreement to spay/neuter their pups, some breeders are choosing to do this before their pups leave.
The following is an interesting thread discussing early/spay neutering:
http://www.yorkietalk.com/forums/gen...n-if-ever.html