Whether you are building a certified system or not it pays to perform an evaluation of your suppliers at least annually at a minimum.
Depending on what service they provide you, there may be critical information that they need to supply you on a regular basis that might be overlooked otherwise. For example if a supplier requires licenses, certifications etc to be able to perform work, you should know that they are current and applicable.
By ensuring that your suppliers have up to date public liability insurances and Workcover certificates of currency, you help lower your risk if something happens. Professional Indemnity Insurance also gets overlooked. We pay quite a lot each year to make sure that we are insured for professional indemnity and yet no one ever asks us for a copy of our policy.
Develop a checklist of critical information and send it out to your suppliers. Keep the information on a spreadsheet and put any expiration dates into your calendar so that you can chase it up when the time comes