The interstate highways have cost taxpayers billions of dollars to provide as straight and flat a highway as possible for the benefit of the trucking industry. So take advantage of what our money has bought and is paying to maintain and stay on these interstates and you will minimize grades and elevation changes and save fuel.
I use Google Maps to get a "bird's eye" perspective on a route and it is easy to determine the terrain and elevation changes and select the "best route". I would just as soon avoid hilly areas if there is nothing scenic along the route as it burns more fuel to no advantage.
On the other hand I would rather deal with grades than dense city traffic if those are my choices for getting from one place to another. I go around the LA basin entirely for example and travel from central California to Boron in the northern desert for example when traveling to southern Nevada or Arizona or even the San Diego area.