It's true that the whole arm should flex. But changing that on a spring already made is fraught with risk. Number one, any material you remove by grinding may reduce the overall power of the spring. Would you be happy with a weaker spring that is working all along its length? As a bow maker, you know that once you start tillering the bow, the draw weight can only decrease.
Where it is working depends on thickness and how it is bent when out of the lock, just like a bow. A recurved bow works hard in the tips because of the bend. So one way of making a spring work all along its length is to control the bend of the working arm. That'll involve heating to working temperature,re-bending the spring, hardening and tempering. Then you hope you don't hear that dreaded "TINK" sound.
From your bow making you know that narrowing the spring will decrease its power in that area proportionally to the fraction moved, but thinning it will decease the power logarithmically. So narrowing is the safest approach, if you do decide to grind. Theoretically, until a stiff area of the spring starts working, you haven't lost any power.
