I don't verify my NTBackup tapes.
When NTBackup performs a verification pass, it uses a checksum comparison to determine if there is corruption in the backup files. If corruption occurs in the data, the corruption may not be detected.
NOTE: Due to the nature of the testing I perform, I run a significant number of backups and restores. I have always been able to successfully restore, but your experience may vary.
NOTE: See section 5.1 of the MTF (Microsoft Tape Format) specification for the algorithm.