If you use the default settings then, yes it's safe to assume all files are garbage. Please not that this only applies to the "Find and remove Junk and Obsolete Files" and "Find and remove broken shortcuts" options on the Clean My Computer page.

However, if you want to be keep the files to be able to restore them at a later time use the "Backup & Delete" or "Zip & Delete" options when deleting the files found.

You can then use the "Restore Wizard" to restore individual files or delete the backup to regain hard drive space.