A few folks have mentioned environmental portraits. This type, and other images in which people are placed into an environment that must be shown, partially dictate lens focal length. Typically, wider lenses are chosen for this type of portraiture... sometimes very wide. As others pointed out, there is no right or wrong regarding focal length.
But... the OP did offer quantifiers to his/her question and that is "minimizing distortion of facial features" at a particular subject cropping factor. As other pointed out, shooting distance controls compression distortion, and how much subject you want in the frame dictates focal length. Shooting angle and lighting add to the final result. The environment, intended effect/message, and facial expression all make a difference on how much distortion you might want. Also, as others pointed out, compression distortion is decreasingly apparent as focal length increases. The bottom line is I follow my previously posted rules-of-thumb loosely but they work very well "for me" given the OP's specific requirements.
Last edited by Old-N-Feeble; 12-11-2012 at 10:32 AM. Click to view previous post history.