some don't stand ever then there are some that needs help to stand (shaving and taping them), some stand on their own later but to be safe i would help them stand. there are other factors that contributes to if and when they stand. like teething, how thick the leather is on the ears and what not....to help them stand i would follow the instructions on this website...
Jon-An's Yorkshire Terriers (How to train Yorkies ears to stand)