100mm,135mm... 200mm all have their detractors and followers. Even macro lenses of 180mm have been used for portraiture.
Modern zooms are right at home in changing conditions and the optical performance will not be a factor for portraiture compared to primes.
One thing to consider if shooting hand-held - with a 135mm lens you really want a shutter speed of 1/250 or higher to avoid shake. With a 120mm or shorter you can use 1/125. Not a problem under studio lights but if you're doing a bit of street photography without hauling a flash around then it can make all the difference.
I was always a believer in the 85mm or 105mm focal length school for 35mm film. Maybe 15 years ago one of my neighbors asked me to take a head shot for her for work. So after I got done with the usual shots using the 105mm in her case, I asked her if she would sit for a few experimental shots. (I knew she would) So I put my 300mm lens on and with a minimum focus of about 13' took a few shots. Then I put on my cheapie 500mm Makinon reflex lens that had macro(?) focusing, and took a few more tight shots. I had the lab print up some 4x6 prints for proofs and showed them to her, and she picked out one of the 105mm shots as her choice for work. But she also like the long lens shots too, even though they changed her appearance quite a bit. (and yes she had a normal nose) She had me make a 5x7 of one of the 500mm shots because she was so intrigued by it.
So I guess you could say that it wouldn't hurt to check with your subject, as they might actually like something different, and experimenting can be fun!
...or do the facial characteristics matter in this determination?
Fat faces, thin faces, faces with large features in proportion to the head size, handsome faces, homely faces...HOW MUCH COMPACTING DO WE WISH TO IMPART?
For 35mm, the 'proper range' runs from 80mm through 105mm. Some even swear by, (or swear AT), the mighty 135mm. But what is REALLY the best way to determine this flexible number in order to optimize the results? Do different faces require different aspects? This is a question that is not parsed too often because the 'literature' gets into the way of challenging theory. - David Lyga
How far away do you want to be? That determines your perspective. Then choose a lens that frames it the way you want without wasting a bunch of film area.
If you try to figure it out with mathematics it is very complicated, because, as you know, angle of view changes as you focus closer. That effect is greater with bigger format cameras.
A head-shoulder picture with a 20x24" camera is taken under conditions where the viewing angle is about one-half that of the angle of view at infinity. A head-shoulders picture with a Minox is taken under conditions where the angle of view is nearly identical to that at infinity.