Bending at the waist is simply a means of maintaining the magic T shape of your arms, shoulders and torso. Stand straight up and draw your bow to hit a target at chest level. You will notice that your arms, shoulders, and torso form a T shape. The goal is to keep that same T shape whether you are shooting at a target above or below you, or want to cant the bow for whatever reason you want to cant it. Bend as necessary to maintain the T shape. If you don't need to bend to maintain the T shape, then don't bend.
People shoot from all different postures. If you find some posture that works better for you than standard form, feel free to use it. It's just that on this forum, we teach standard form. No doubt standard form will continue to evolve, and will probably be different in some respects in 10 years than it is now. And, people will come along, like Rick Welch, who are successful shooting and teaching styles that vary in some respects from standard form.