No, they don't have to go to zero. Most don't. If I'm making a light weight bow, do the math, and the tapered lams will be too thin on the thin end, i'll use less number of lams and make them thicker. But it all depends on how it breaks down.
Stack is measured at the butt end regardless, and the lams are tapered to whatever degree you need to get the limb action you want.