If you want stimulus focused into your upper back then move lower as little as possible. Focus on the rowing movement. Pure rowing machines also have your back fixed. So nothing wrong with keeping back straight. I do find it easier to move heavier weights when getting assistance from the rest of the back. May be because of that you see folks move their back.

Buuuuuuuttttttt

At the other end is the flexion row where you not only move your back but flexing it allowing the spinal erectors do some work. I love this variant to compliment regular stiff back rows.