The Apple guidelines are:
QuoteData Handling Categories
The com.apple.MobileBackup file attribute introduced in iOS 5.0.1 is the "do not back up" attribute used to specify which files or folders should not be backed up, regardless of their location in the file system. By using this new attribute and placing files in specific directories, files can be described as one of four different data categories.
Critical Data
This is user-created data or other data that cannot be recreated. It should be placed in the <Application_Home>/Documents directory and should not be marked with the "do not backup" attribute. Critical data will persist in low-storage situations and will be backed up by iTunes or iCloud.
Cached Data
This is data that can be downloaded again or regenerated, and the absence of this data will not prevent your app from functioning as the user expects in offline situations. Cached data should be put in the <Application_Home>/Library/Caches directory. Cached data may be purged in low storage situations, and is not backed up by iTunes or iCloud.
Temporary Data
This is short lived data that the app needs to write out to local storage for its internal operation, but that is not expected to persist for an extended period of time. Temporary data should be put in the <Application_Home>/tmp directory. Files in this directory may be cleaned up by the system. Files in this directory are not backed up by iTunes or iCloud. Temporary data files should be removed as soon as they are no longer needed to avoid using unnecessary storage space on the user's device.
Offline Data
This is data that can be downloaded or otherwise recreated, but that the user expects to be reliably available when offline. Offline data should be put in the <Application_Home>/Documents directory or in the <Application_Home>/Library/Private Documents directory (see QA1699 for details) and marked with the "do not backup" attribute. Data stored in either location will persist in low-storage situations and the "do not backup" attribute will prevent iTunes or iCloud backing up the data. Offline data files should be removed as soon as they are no longer needed to avoid using unnecessary storage space on the user's device.