I think the Basecamp app is even better, although still pretty much like MapSource. To my knowledge, even the high-end GPS's (I have the Zumo 660) don't have an easy way to tell it what roads you want to be on, other than to manually select a lot of waypoints between your departure and destination points, which will force it to plan out the route using the roads you want.
But as Cigarski says, it's easy to select any roads you want on Mapsource or Basecamp by using the "Select" tool on top and then simply "pulling" the route line onto the road you actually want to use.