Although its not necessary, you can flush it occasionally when you add or remove images from the phone.
You may experience some tiny lag in phone's functionality till it creates the catche again.
There's no other risk whatsoever.
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Die hard Nokian, willing to help other Nokia Users.
Current Handsets - Lumia 900, Nokia 311, Nokia C3-00, Nokia C1-01, Nokia 6500-s.