From reading the Daybooks I get the impression that some of the women did not want their faces in the pictures in order to maintain/prevent problems with their social standing. But there is an example of an early Weston nude in the Aperture "Nudes" (Weston) that does not have the face/head cut off, so not all of his models minded.
That said, he certainly seems to have been looking for pure form and including the face may or may not have been a decision that was influenced by the model.
However, Charis Weston notes in her comments that after a certain point he began to always shoot entire bodies. And I suspect that the chronology of the nudes he did of her would show that the earlier ones before they were married did not include her face as much as the later ones. Also, at that time he was no longer photographing Bertha (the dancer) so perhaps he wasn't "seeing" parts of bodies as complete compositions anymore?? Who knows? It doesn't really matter since the man was able to make fantastic photographic art no matter what he did.
Best,
Will