You are only obliged to complete a Customs Declaration Form if you are bringing goods or possessions that are subject to declaration. However, it's not unknown for customs officials to insist on you providing a declaration, particularly on popular trades routes such as flights from Turkey. The regulations covering what items you are obliged to declare are listed here. If you are uncertain whether or not items in your luggage should be declared, it is always better to make a declaration, then you can rest assured that you will not have problems when leaving the country.
If you do fill out a customs declaration, then make sure it is stamped by a customs official, and keep it safe until your departure. As with other official documents, we recommend that you make photocopies of your customs declaration in case of loss or theft.