At one of the customers I am working with quite closely, all buyers are allowed to create new (unapproved) supplier organisations. A central team takes care of the approval and checks to see whether the supplier organisation already exists.
At another customer I work with they are implementing a global supplier registration process. There the approval flow will be driven by "Country" in the postal address of the supplier. Depending on the value of the "Country" field the system will route the approval request to a specific regional supplier managment group within the company. This is accomplished using the Team Member Rules file in the supplier workspace template.
This is a centralized function within the Supply Chain organization. A supplier maintenance portal on the web is used by requestors to provide the necessary information regarding new suppliers to be set up, or for changes to be made. Prior to being added, organizations are researched and validated, including OFAC scanning, and the existing vendor master list is reviewed to ensure supplier is not already set up, to avoid duplications.