I've always used Pantene shampoo and conditioner on mine. I use baby shampoo on their faces. After rinsing well, I spray them with Johnson's Baby Detangler. Mine haven't had any skin issues. Some people have yorkies with skin issues and that could present a problem.
This is from the YTCA:
Yorkies are a “single” coated breed, meaning that they do not have an undercoat. Their coat is very similar to human hair and we suggest not to use human shampoo. A dog has a different ph balance then humans and using “people” shampoo can result in dry, itchy, flaking skin.
If you do use a human shampoo, please make sure it’s a good quality. Remember to completely rinse the hair after shampooing and conditioning. It is also best to remember to always brush the hair on a Yorkie that has been sprayed with a light mixture of conditioner and water. Never brush a Yorkie coat when it is completely dry because this will damage the coat.
http://www.ytca.org/grooming.html