Should I sign a W-9?

A W-9 doesn't provide proof that someone carries any insurance, license, bond or anything else; it's intended purpose is to doent Social Security Numbers of sole proprietors and to certify that "backup withholding" does not apply. Inform the contractor that you will gladly provide him with the form prior to the payment for any services. Let them know that it is against your policies to release that information without due cause. Also let them know that you will provide them with certificates of insurance, bonding, and licensing if they wish to verify those issues.