Business model: least work routing for truckers, big RVs,
people towing trailers, etc. Pitch as a fuel-saver,
promote with ad-supported local and special-interest ads.
Support click-n-drag re-routing as Google Maps does now.
Note that while many truckers travel recurring routes,
some pick up ad-hoc LTL (less-than-truckload) shipments
and must replan routing with nearly each shipment. For
fun, demo using:
Tucson, AZ -> Tucumcari, NM
Tehachapi, CA -> Tonopah, NV
An ideal route isn't best characterized as "flat" - I agree that "least work" (in a mechanical sense) is better. In reality it is as you say however - a personal trade off - some people just love hills too :)
This way, you could make tradeoffs between distance and effort.
I'd happily travel an extra flat mile to avoid a .25 mile hill with a 9% grade.