Never heard of us using 757 winglets, but we redispatch quite a bit to save fuel when able. They are really pushing us to redispatch everything international with 4+ hours of flight time. There a few factors that make it impractical sometimes, such as ETOPS add fuel, weather, ATC issues, or old airplanes overburning and landing short in TPE. Many times you will put an aircraft below the FOM suggested 75 minutes of landing fuel or even below emergency fuel at alternate. Some captains and dispatchers don't mind, but some some do.