Dealing with corruption: file-level backup vs. pg_basebackup
pg_basebackup and other methods like pg_dump might not actually finish due to the corruption, and you really need that backup in that case. Being independent of PostgreSQL and its tools, tar is not as likely to run into those issues.

In the case of corruption, that backup will be an invaluable insurance policy when you try to recover your data or do forensics, so playing it safe is always the best way to go.