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With my four-ton pickup (and more so with 35' travel trailer in tow) I enter a highway with the intention of of using terrain to help the last 10-15/mph up to set cruising speed.
That is, I'll accelerate up past the 45-mph legal minimum, but I wish to use any sort of downgrade to get it up to the usual 59-mph and set the cruise control.
Terrain and traffic don't always cooperate in this, but it does lessen the engine load when those other factors allow for it.
I prefer that commercial traffic be a quarter-mile back, and cars less so.
Alternately,
One uses the brakes, not the throttle, to merge onto an Interstate. Ideally one is at or above the legal maximum well before the end of the ramp (most entrances are downhill for this reason). In this use of a downgrade the penalty is minimal compared to the safety advantage.
Choosing the point of merge means more in my experience.
As to rolling terrain I find that setting cruise control back by two mph obviates the penalty of its use.
With hills, I let it run out some. Maybe five mph downgrade. On an upgrade, more of a change in speed is acceptable. But not to the point of having traffic jam behind me (relative to flatland steady-state).
As I run 10k miles monthly as a pro driver, spacing with other vehicles is paramount. I can't do a thing about tailgaters (less than 100' behind me at speed), but there's no way I'll allow things to jam up ahead of me (less than 200').
So, yeah, aceelerate when there's less penalty, and don't exceed 80% Engine Load in a climb.
Frankly, we most of us run so few highway miles annually (this based on engine hours) that what we do out there past a sensible set speed (60 is The Wall) doesn't mean much to the calendar average.
IOW, with no lane changes and no use of brakes, 58 or 62 or 66 don't mean much. It's tire wear and other factors pertaining to longevity that factor higher.
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