i bathe my pup once every 7-10 days b/c her fur does start looking nappy. it really does make a difference b/c their hair is more similar to human hair than normal dog hair. i use cheapy puppy shampoo right now, i think when her adult coat comes in, i will switch but she doesn't seem to have any adverse effects to the puppy shampoo so i just keep using it.
its good to get them use to grooming young though so that they will learn to tolerate it when they are older. this thread has a lot of shampoo suggestions:
http://www.yorkietalk.com/forums/yor...o-yorkies.html