Guess it depends on who wants to get back the files....
For the usual computer user a simple deletion of the files is enough.
However, if you expect some officials to get interested in your HDD (for whatever reason), then a simple deletion will not keep them away from bringing the files back.
In general, for people concerned, it would be necessary to use an eraser program (best one which offers the DoD deletion mode) to get the maximum success in deleting files (though that doesn't mean that this method is 100%).
