The AKC standard for the yorkshire terrier says that the "Hair should be trimmed short on tips of ears" (
American Kennel Club - Yorkshire Terrier). For those showing their dogs, this generally means the top third of the ear. If you are not showing your dog, you pretty much can groom him as you please. Some people like the keep the ear "fringe" and like the look of it, whereas I prefer the clean-line look of a trimmed ear. Sometimes, a yorkie may have so much hair on their ears that it causes them to droop a bit. Using the clippers to remove this excess hair can often remedy this problem. Hope this helps!