You want to keep your toes pointed throughout your flutter kick to ensure maximum propulsion and minimum drag. Do this by pointing your toes at the back wall you are swimming away from versus pointing them at the bottom of the pool.
If you have problems with flexibility try kicking with a pair of flippers on. The flippers naturally create a pointed toe and will assist you with flexibility in the foot. Make sure to practice with and without flippers so that your kick doesn’t come dependent on them.