Sarbanes-Oxley legislative rules dictate that audit-related documentation must be kept for 7 years.  To that end, when the question arises "should I keep this?" the answer is going to typically be yes.  Vibato recommends storing all of your internal audit documentation in a bankers box by year and marking on the outside with a Sharpie that the contents should not be destroyed before X year (X year equals this year + 7).