I think the IRS technically only requires a completed W-9 so fax and scanned-emailed copies of completed W-9s are OK, but a requester might want to play it safe and require it in hard copy.
It is fine to fax or email a W-9 to a requester if they are okay with it. Do keep in mind, however, the security risks associated with electronic methods; a hard copy may be the safer option.
A W-9 must be signed by the employee. After it has been signed, it can be faxed or emailed to the employer. However, email is not a secure transmission method.