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As long as you haven't selected to backup to icloud, then it will automatically backup to itunes everytime you sync.
 
Do you have settings - icloud - storage and backup - icloud backup on? If not then you will perform a backup to itunes everytime you sync.
 
Do you have settings - icloud - storage and backup - icloud backup on? If not then you will perform a backup to itunes everytime you sync.

I have it off, but for whatever reason my iPhone does not backup when syncing.
 
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