You should also back up /etc since this often contains any modifications you
may have made during the installation process.
I do:
Every system file (under /; don't forget, for example, /boot/grub/menu.lst)
modified is stored under RCS.
A nightly cron job e-mails me about files still checked out.
A weekly cron job gathers the RCS directories and stores them on a back-up
system.