I used to retain the last 2 Full AND Transaction Log backups pending verification of a RESTORE into a shadow DB however, after listening to sob stories from the IT geeks that my disk usage had risen from 4 GB to >16 GB, I dropped down to retaining the last Full Backup only.

I had an erraneous backup on my main DB recently which only came to light when I attempted a RESTORE into my shadow DB. It was a nervous couple of hours before I could RESTORE another hastily triggered Full backup, an event that highlighted how precarious and inadequate my maintenance plan was.

I've since returned to retaining my last 2 Full AND Transaction Log backups.