I believe you are right to an extent, the purpose of the 'winglet' is to reduce the magnitude of the wingtip vortices. By reducing these you reduce the 'induced' drag and that equates to less thrust required and therefore a fuel saving.
Some aircraft such as the 777 have such well designed 'raked' wingtips that there was never a need for winglets.
With your answer, they don't reduce 'trailing edge vortices' but 'wingtip vortices' and I have never heard of 'pressure' drag.
Last edited by Ollie Onion; 16th June 2012 at 10:53.