Yeah, i agree it's not a best practice, but whether to look at CB or OB last seems less important than the other parts of the shot. By the time you're pulling the trigger, all that aiming stuff should be over with. Now it comes down to your stroke. Your not steering your hand to the aiming point like you'd steer a car around a curve. The path your back hand is following is determined by your fingers, wrist, elbow, shoulder, and head, and maybe your knees and legs if you're a 'mover'.
Judd Trump is a good example. A purist would say he aims wrong, and strokes wrong (not down his aiming line), but here the two errors cancel each other out. The two errors are consistent and he's a great shooter. Here's a link to good example where the camera is looking back down the cue, both on this red (at 6.50), and on the final black.
Trump 147 in 2022 And these are soft shots, It's even more obvious on harder shorts.
Trump says he didn't know he was doing this until someone told him, and the consensus is that it's not worth trying to fix.
So in his case, his stroke is correcting for his aim. He's a very good shooter because he's very consistent.
If I properly prepare for a long straight shot, the aiming should be trivial, and so if I miss, I assume that my stroke was wrong, and usually it is.
I've tried both CB last and OB last, and I didn't think there was much difference between them in terms of making the shot, but with OB last, my pace was usually better, and my resulting position was better.
The only time CB last seemed to be better for me, was when the CB was against the cushion and I had a very tiny target to hit.