Should Cache Files Be Backed Up?

I have to believe that this has been covered somewhere but I haven't been able to find it in my searches.  When doing a system backup the documentation seems to recommend backing up the "moodledata" directory.  That directory contains cache files and session files that seem to change regularly (frequently) and on our system they take a huge amount of time/space to back up. 

Is it necessary to keep these files backed up or are they just things that will be created as needed when the site is in use?

Example: moodledata/cache/casestore_file/default_application/core_htmlpurifier/ (subdirectories numbered from 001 through FFF!)

Thanks

Tasha