In my experience, the ACA gives highest preference to paved roads, but they will occasionally use short segments (a few miles or less) of unpaved roads to avoid heavy traffic on the paved road.
When considering among paved roads, the ACA will almost always pick the lowest traffic roads, even if it considerably increases the hills and distance (up to 50% longer) and sacrifices the shoulder. Most people who deviate from the ACA routes do so to decrease the hills and distance, but almost always at the cost of more traffic.
If your goal is to avoid traffic, and you're willing to use shoulderless roads to do so, then my advice is to stay on the ACA route exactly. Almost any deviation will increase traffic.