From the instructions for the W-9 form and the rules for small businesses working under an independent contractor agreement:

Forms and Associated Taxes for Independent Contractors

Form W-9

If you’ve made the determination that the person you’re paying is an independent contractor, the first step is to have the contractor complete Form W-9 (PDF), Request for Taxpayer Identification Number and Certification. This form can be used to request the correct name and Taxpayer Identification Number, or TIN, of the worker. A TIN may be either a Social Security Number (SSN), or an Employer Identification Number (EIN). The W-9 (PDF) should be kept in your files for four years for future reference in case of any questions from the worker or the IRS.

[According to this, we MAY have to file a new W-9 every 4 years but not annually.]

Another place in the instructions for filing the W-9 indicates that we ONLY have to file a new W-9 with our clients if there has been a change to our corporate status:

You must provide updated information to any person to whom you claimed to be an exempt payee if you are no longer an exempt payee and anticipate receiving reportable payments in the future from this person. For example, you may need to provide updated information if you are a C corporation that elects to be an S corporation, or if you no longer are tax exempt. In addition, you must furnish a new Form W-9 if the name or TIN changes for the account; for example, if the grantor of a grantor trust dies.

 The above paragraphs are what I will be sending out to clients who request a new W-9 be filed annually.  It isn't required by the IRS and a company's accounting department shouldn't be making busy work for vendors.