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I haven't tested this so much on flat ground, but even on inclines, if I downshift from 5th to 4th, I lose about 25-30% fuel economy. I have a real-time gauge, and on one hill I've watched this effect closely. At ~ 35-40 mph, I'll drop from ~20 mpg to ~15 mpg... same speed, same incline. For reference, the rpm shift is from ~2,000 to ~3,000 rpm.
I would say, as long as you are cruising, you should try to stay in your highest gear. Sure, if you must pass or accelerate, you'll probably want to downshift. From the behavior I've observed in my real-time fuel gauge, I always get far better mileage while cruising than I do while accelerating... regardless of which gear I'm in and regardless of whether the engine feels sluggish.
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