How much additional proofreading is appropriate?

The verification process catches problems that ABBYY FineReader notices, but it does not know what it does not know. It may have high confidence that it recognized a letter correctly, but nonetheless be 100% wrong.

Given the volume of pages involved, it is not practical to read every page word-for-word during the OCR process. At the same time, there is much to be gained from quickly correcting any obvious errors you notice. But bear in mind that formal proofreading will be done later, after the OCR process is complete.