Something to remember is that since a Garmin Nuvi is just a computer that derives its recommendations from an algorithm, if you have it set up for shortest route and it finds a route that is shorter by just a little bit, it will make that recommendation. Mine has a detour function which directs the GPS to find a plan B. Compare the first recommendation to the second and you may find that staying on a main road only costs a mile or two over a great distance -- stay on the main road or at least know why you are leaving a main road. Saving a minute or two off a long trip just isn't worth the security of a main road.

I do the same thing using GoogleEarth combined with the Garmin/Mapsource software. Mapsource makes this easy by providing a "View in Google Earth" function. Compare multiple sources and look at the date of the source you're using -- things change.