Do I have to have all back flow devices certified?
My company is low risk, RTE baked goods
Generally speaking you only need "certification" if required by local, county, regional, etc regulations.
While you do need backflow devices, most facilities place them on their monthly or quarterly PM plan - the Auditor will ask to see your backflow records.
If I remember correctly (and this is going back aways when I was an Auditor) when I was in Chicago, Cook county required a specific company to do the certifications and issue certificates. In that case as an Auditor knowing that certs are required I would be asking for the copies of the certs and records on maintenance, etc.