With Schatzie I bathe her every 3 days and she does go also to a professional groomer to manage her anal glands (i wont touch that, gross), clip nails and trim hair as needed.
I guess it depends on the Yorkie, what type of skin it has regarding allergies, dry, itchy, other issues, etc, or what type of enviornment the Yorkie is in: for instance some Yorkies love to play in mud, go hiking, dig in the dirt, etc.
I know there is a lot of controversy over the issue of bathing and how often, what products to use on the Yorkie, etc. So I'm just guessing it's a personal issue on what is best for your Yorkie.