I've had a few customers in my cue repair business over the years ask for a marker line on the ferrule to help with their aim.
I twist my cue, but its not intentional, and I don't think my mechanics are flawed. I first discovered it when I put on Joe Tuckers' 3rd eye stroke trainer on my cue. After about 10 back and forth strokes, it had rotated 45 degrees or so. My wrist did not twist, my stroke was straight, and my fingers did not twirl the stick.
I do play with a rubber 3 cushion grip on my cue, and I think what was happening was the extra friction of the grip against my fingers would somehow cause the slightest cue rotation. And after several strokes, and the 3rd eye on the cue, it was obvious the cue was rotating in my hands.