A full SEO Audit by an experienced Professional SEO analyst is essential. Without SEO Audits things will slip for sure. Every one of us always miss out the odd tags here and there, maybe a page title or two. Alt tags. broken links and so on. You need to be perfect every time or Audit on a regular basis. No one is perfect. So to keep up to date it is advised to complete a full site audit every 3-6 months for content websites like blogs, for small local business sites every 6 - 12 months depending on the competition in your niche