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The owners manual for my Honda says drive in the highest gear that still allows the car to accelerate, which is the general rule I follow. If you can still accelerate then the motor is not being overworked, and you should be safe. If you are losing speed at high engine loads, it's time to downshift.
As far as MPG strategy, the most fuel efficient gear is the highest one. If you are going any speed, say 30 mph, higher gears will be more fuel efficient than lower gears because of the lower RPMs. The higher load required in the higher gears doesn't matter, as long as you are not going high enough to enrich the mixture (~90% load, WOT).
Adding to the confusion is BSFC. Keep in mind that the ideal RPM and load range set by the BSFC chart might be the most power efficient, but it is not necessarily the most fuel efficient. People that do P&G accelerate based on the BSFC chart because they want to maximize the efficiency of getting back to the top end speed so they can glide.
People that use the DWL (driving with load) technique need to use the lowest possible load for each situation, because lower loads = higher MPG's. We are not concerned about the BSFC while cruising at a steady state, only when accelerating. So going up a hill on a highway you need to find the highest gear you can maintain your minimum speed in.
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