Rope twist

The Edelrid knowledge base has a video showing a belay device causing twists when the rope axis is not perpendicular to the carabiner axis. How closely do folks usually pay attention to keeping the rope perpendicular to whatever the rope is running over?

I dont really understand what youre trying to say, but i can tell you that i would happily leave a bail biner to avoid twistmaxxing my rope. 

If you perfect your Munter hitch technique, you'll understand and pay attention to this phenomenon. You can observe it directly by watching the rope pattern as it feeds out of the belay device. The radii of curvature of the carabiner also has an effect. Different rope/carabiner/belay device combinations produce curiously different behaviors. Maybe I'm imagining things, but I think the behavior changes with humidity/wet ropes. 

As noted above, this phenomenon also bites climbers when pulling a rappel rope through chain/ring/carabiner that is constrained so the rope axis (cylinder/radial symmetry axis) as it runs through the ring (radial symmetry axis) is not running perpendicular to the ring axis. Hence additional rings/carabiners/chain on many rappel bolts.

This phenomenon is worthy of study and a would make a nice journal article.