One expert bootfitter I know has stiff feet and ankles and prefers softer boots buckled tightly to his ankles. Two other expert bootfitters I know have very flexible feet and ankles and prefer 150 flex boots that have some ankle room that they use as an indicator to feel whether they're against the front or not.
Add personal physiology and preference to the boot selection equation. I always loosen the top buckle and Booster Strap in powder and bumps to let the boots "self-level" under me in powder and provide shock absorption in bumps. I ski better with the spoiler spacer gizmo between the cuff back and the liner...I've tried with and without, and the slight added tilt works for me.