The Boeing 747 anti-ice is actually a de-icer. You should wait until there is a build up of ice on the wing leading edge and then switch it on for about two minutes to break the ice away. Switching it on so as to prevent ice forming can cause water to run back and freeze on the upper surface of the wing.
Having said that, the 747 wing is not prone to icing. In 32 years flying the 747 I never used it, and that included over 100 approaches into Chicago in the winter.
Dave