Not Backing Up Files That Don't Change In A WordPress Install

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http://serverfault.com – I have a WordPress install and I'm wonder what is generally best practice when it comes to back up. I'm asking here instead of WordPress.stackexchange as this question is not only a WordPress question but a general backup question. Is it okay to NOT Backup files that won't change. For example in my install I have files such as the theme and plugin directories which won't change and I'll have a copy of these files in there original state. My thinking is then that they shouldn't need to be backed up as they can just be restored at any point. Is this good logic? Or is it better to back up eve (HowTos)