depends on your definition of portrait.

but if you are talking about a close up head and shoulders then there are a
few things you can do.

keep the 'plane' of the face parallel with the film plane.  imagine a sheet
of plastic on the face with a hole for the nose, it would rest on the chin
cheekbones and forehead (note: not really I know but work with me)  The less
the head is out of true with the camera the less distortion.

center the head and crop

don't shoot so close up, if you use a heavy vignette you can move back yet
keep the concept of a close up going. shoot half lengths, full lengths.
Heck I use a wide angle for family portraits on the beach, the subjects
might just be small compositional elements within the landscape but hey, it
helps sell wall prints.

if you do move back, avoid any pose that puts hands or objects in front of
the face, IE: a typical obvious distortion is having the subject sit with
one knee up and the hands clasping it, sometimes the hands would be as large
as the face.

This reply is echoed to the z-prophoto mailing list at yahoogroups.com

Possible yes but don't get too close.

Go to the library. IMO CB was not primarily a portrait photographer but
YMMV.

Sign,
me

PS: Something looks fishy?