I think you are confusing icing condition for use of Engine Anti-Ice with Airfoil Anti-Ice.
There is probably no aircraft in the world where you wait until you have Ice until you hit the Engine Anti-Ice button. You want to be safe, remember AUA F70 in Munich with double engine failure (for other reasons, ok).
Airfoil Anti-Ice (also called Wing and Tail Anti-Ice) uses much more engine energy (bleed air), so you want to wait with that a bit longer. You only use it when you have actual ice on your Ice indicator (can be an mechanical-electronical devise or your eye).
hth,
Dani