Tagging is important. Creating tags helps us to attach our business logic to the hardware we're creating on the cloud. There is a difference between telling your CFO that we're paying for 10 r5 instances, and telling him you're paying for 10 DB servers for the app he knows. If you haven't tagged your environment yet - leave this post now, go tag your resources, It'll still be here later :).
But what about resources that are untagged?
Not many know, but in AWS there is a dedicated cost tag (which is not accounted for in your tags quota, and you won't be able to find it anywhere in the console, nor in the tag editor - only in the cost reports), that will apply only on eligible services & resources after specific events. Once you enable it in the cost allocation tags, it'll be available in the cost reports.
The Tag structure:
key = aws:createdBy
value = account-type:account-ID or access-key:user-name or role session name
Granted - it's not an overall solution that replaces the responsibility of the engineering teams to tag the resources, but it does give you an additional layer of visibility on your resources, even in the rare case that user-defined tags weren't applied to them.
For more information you can read all about it [here]