I've experimented with a CB-last eye pattern, but always have best results with OB-last.
In my opinion, CB-last robs the shooter's brain of vital feedback data: a clear picture of the cue ball's path (including deflection and swerve), and most importantly, a clear picture of the "moment of truth," contact with the OB. Why? I believe it's due to
saccadic masking, where the brain "selectively blocks visual processing during eye movements in such a way that neither the motion of the eye (and subsequent motion blur of the image) nor the gap in visual perception is noticeable to the viewer."
In other words, as our eyes rapidly track from CB up to OB immediately post-tip contact, our vision goes effectively blank, and we may have trouble getting a "lock" on the OB prior to CB contact with it. The firmer (faster) the shot, the more difficultly we'll have in seeing the CB roll path and contact... and if that data is not made available to our brain, our subconscious cannot learn from each shot. At most shot speeds, it's literally impossible for our eyes to lock on the CB and track it in motion, and our attempt to lock on the OB position to see CB-OB contact will either be late or inaccurate.
In summary: Locking on the OB last gives us the best "quiet eyes" position from which to learn from the most important aspects of our shot – the initial track of the CB, any swerve along the way to the OB, the exact position and spin of the CB at contact with the OB, and the resultant path of the OB.
Personally, to maximize the feedback from every shot, I try to observe the fractional overlay of the two balls at contact (and notice if it matched with my aim line and estimate of deflection and swerve), then observe the spin-induced or cut-induced throw (if any) of the OB, then of course observe the OB path to the pocket or other aim-point. When I really bear down and observe these fine details, and stay down on each shot absorbing them until the balls stop rolling, I find myself in "the zone" and my pocketing percentage and position play are noticeably improved.