There is a synergistic effect from using both Rogaine and Propecia together. This has been confirmed by many of the top HT practices from experience with patients.
The reason Rogaine is sometimes referenced as only working on the vertex is because this is the area the company carried out their extensive clinical trials. So they can only legitimately market the product along with the stats from their trials in its application to the vertex.
However many people do have success with hairline and mid core so it is always worth a go as you never know your luck.