Personally, I delete Terminated instances after I have determined what went wrong and taken any steps I needed to manually clean-up records and/or restart instances.

In the case of Completed and Cancelled items, I just rely on the built-in Data Maintenance options to clean them up.  I have it set to remove instance data after 2 years (730 days) and instance statistics after 5 years (1825 days), I also have it removing uploaded files after 2 years (730 days).  Any items I want permanent record of have been sent to the Repository, so the Forms history is more for reporting and research, and in over 5 years of using Forms I haven't needed to researching anything older than 2 years, and rarely older than 1 year, so the 2 years of history seemed very safe.