I would wait until it gets warmer. Yorkies dont handle being very cold or very hot. I would just try to brush her to keep her hair from tangling. At that age a good brushing 1-2x a week should do, but as it gets longer you will have to brush more to keep her coat in good condition. Fisher enjoys being groomed everyday. I bought a little brush and brush him until he is snoring lol
To me their coats are so much more beautiful longer. Fishers isnt as long as it will get, but when it does reach its full length I will prob. trim it a littler for better control