I look at the OB first to set my aim line. 
Once my bridge is planted, I look at the CB. 
1.  If my aim line was correctly set to start with, I don't need to see the OB last.  (It is not moving from where I last saw it!)
2.  Monitoring the CB last (or, better said, my CUE TIP) helps to ensure I hit with the 
     A. chosen English, 
     B. speed, 
     C. follow through, and
     D. I can monitor my stroke for straightness
In other words, I have better success monitoring and executing a good stroke on the right contact point of the CB than I do by looking at the OB last.
But hey, you do what works for you.