The main reason to start at a low temperature such as 130°F is to dry the outside of the casings so that the sausage will develop a nice brown mahogany color. During that first hour, smoke is not applied. When the smoke is being applied it is best to keep the temperature at 140°F to get the best smoke flavor. After the smoke is applied, it can be bumped to 170°F.
If you go to this site:
Cooking SausageThe authors of that site state that when applying smoke, during that time you can slowly increase the temperature to 160°F; but he gives no guidelines of rising the temperature during that time. My feeling is, if you only applying smoke for two hours; is it worth going out every 30 minutes of so to increase the temperature by about 5 degrees.
PS
I forgot to mention that drying the casings also improve the smoke flavor.