I see no point why winglets should be most efficient in cruise; as I remember my aerodynamics lessons winglets are best at reducing induced drag which is increasing with decreasing speeds and increasing angles of attack.
Also, winglets are not only useful for long-range airliners; just look at the canadair regionaljet.
A look at my Airbus 320 operating manual shows that with ONE wingtip fence (small winglet) missing, the fuelflow increases by 1.4%, and the take-off weight as well as the approach climb limiting weight has to be adjusted by 4 %.
Quite a change at the low-speed range, I think