You can almost certainly ignore
A good case can be made for ignoring
/var/log (and any other logging directories),
/var/cache if you have it, and possibly portions of
/var/db (though you have to be careful with
/var/db: Sometimes really important stuff gets put there...)
Beyond that you probably want to do a backup, wait a few days and do another to see what changes over time. If you see a lot of "junk" in those backups you can tailor your exclusion list more carefully.
Once you have selected the stuff you want to back up & tailored your include/exclude lists make sure you do a proper restore test -- take a machine from bare metal and go through the process required to get your data and software working again, without touching the original machine.
If you can't manage that restore with what you backed up you really don't have a backup...