The answer depends on your machining strategy, are you using a corner rounding cutter?
If using an End Mill then I'd go with linear cuts with a square endmill to rough it out to fine steps quickest then vertical passes with a ball end mill to best smooth the corners.

At the size transition step can you get away with a groove around the part? Thus allowing it to fit flush to the step edge but eliminating the machined transition from the rounded corners at the step size transition?

- Nick