Yep. Definitely, do it once a year, and make a big fuss of it.
After all, if you do it every day or week, you'll be re-using your backup tapes[1]. If it takes a while to spot the malware, it might have infected the backups, too, and you'll spend the next year working through them to find the most recent clean one.
[1] Or substitute your choice of more modern media.
