Another option :
Create a simple Audit universe report that retrieves all the reports (Crystal,Web Intelligence and if you have other types of applications than add them as well ) that been viewed ,run and refreshed in the last year or more if you think it's relevant.
Than you can make a simple day difference calculation that can display the last date a report was activated and how many days passed till today.
You can define according to your guide lines that if a report wasn't activated more than X days - it's importance is low and not critical for the users and groups - either way you can identify those reports that way.