I do guess it will require but I have never used a heat sink before, so I am not sure.
Depends a bit, 5W on a naked TO220 is pushing it in the absence of airflow, and I would probably screw the thing down to some suitable bit of metal if it was to be sustained for more then a few seconds.
What does your parts datasheet say about Tjmax, Rjc and naked package thermal resistance to ambient? How hot is your ambient? Can you mount the thing right down near the board and use a honking great copper pour?