Here is an interesting article on the pro's and con's for spaying and neutering. You also have to consider what is in the best interest of your dog's health.
http://www.naiaonline.org/pdfs/LongT...uterInDogs.pdf
As for baby teeth. From my experience they usually lose all their baby teeth before they are a year old but there are exceptions. The small yorkies seem to be more prone to retaining baby teeth and I think that might be because they don't chew as much on hard things. All the permanent teeth do not come in until the puppy is 6 to 8 months old.
Here is a very interesting article on puppy teeth from a Pom breeder:
Bi-Mar Pomeranians - Teeth Cleaning