Since I dont yet own a Yorkie, I'm not going to comment....and also because my opinions are so varied on this topic.
But I just have to pop in and say that, though it's true in many ways this is repeated information, I find this to be one of the most informative threads! Responses have been to the point and tactful at the same time, which makes it really easy to learn from and understand different points of view. I have gleaned some new ways of looking at things from this thread, and look forward to more. Thanks to everyone!