I have always been an OB last person. However, as my game has developed, I've had to take a slightly more nuanced approach. Here are my thoughts:
The point of looking at a target last is to improve your accuracy in hitting that target.... That is to reduce the potential for error. If you look at the OB last, by and large, you should be able to hit your intended target on the OB with more accuracy. Conversely, if you look at the CB last, you should be able to hit your intended target on the CB (with the cuestick) more accurately.
The problem is, you can only look at the CB or OB last...not both (at least not in proper focus). So which to look at? I think the answer lies in which is the more important target to hit on any particular shot. Is it more important to reduce the error on the CB target or the OB target?
For a typical medium length shot with near centerball hit, even a slight error on the OB target can result in a missed shot...whereas a slight error on the CB target generally won't result in a missed the shot (due to deflection or throw) but can result in a bad leave. On the majority of shots, the former consequence dominates the latter. And for longer shots, this is even more pronounced, because a small error on the OB gets amplified even further.
However, for other shots, errors on the CB become the dominant factor; masse's and Jumps are two obvious examples. Small errors on the CB can result in a foul...a much less desireable result than simply missing the shot, or not getting position.
Now, take the example of a very long extreme draw shot where the OB is in the jaws. Here is an example where you have a pretty large margin of error on the OB, however a minor miss on the CB can result in a miscue. I used to always look at the OB in this situation, but I've now come to the conclusion that often CB last makes more sense.
In most cases, looking at the OB last probably makes the most sense. However, for some shots (especially those where CB accuracy is paramount) CB last is oftentimes the best way to go...IMVHO.