Human beings are keyed into the human face and what is considered natural vs unnatural, which is why a face/head which is slightly off feels so wrong. If you are looking to to achieve a semblance of real world proportionality, I would jump in and say that doing studies from reference is important to get the normal proportions of the human head down pat. That being said, there is a place for the stylized and uncanny, it's just a matter of experience recognizing what will be uncanny and what will be endearing. If there's anything I've learned from being a professional artist, and subsequently ceasing to be a professional artist, it's that the message you are trying to convey is more important than the technical execution with which you convey it. The message itself is more important.