Generally, the Wordpress files change extremely infrequently (only after updates). The files are always available for download from the Wordpress website, so it may not be worth the time and storage to do.
If your users upload files, you may have to back those up. However, you should consider that most uploaded files (images, movies, sound, .zip files) have an extremely low compression ratio. Backing up 1 GB of these types of files will likely require 1GB of space, not to mention the time downloading and uploading the files. A much easier solution would be to require your users to use image, video, or file hosting websites which are always online and never need you to back up the files.