Hmm, how do you mean?
If you know the subject well, good enough to draw him or her by heart(although this seems unlikely), no. If it's an imaginary subject... No.
Actually, I think a high-quality image of the subject will do as well as the posing subject. These days posing for portraits seems a little unnecessary, besides from training for the artist making the portrait.
In general: Not necessarily anymore, because you can also use a picture of the subject in the right pose?