I think in most cases, driving with the cruise control turned on is better for the gas mileage. The car is going to get better mileage the closer it stays to a constant speed and rpm, so cruise control is better in that respect. However, if you're driving through a lot of hills and causing the transmission to shift a lot the engine rpms are going to be all over the place which is inefficient - remember, nature prefers to resist change. You may be able to reduce the amount of shifting by changing your speed slightly or by turning off the overdrive.

As far as shooting for minimum rpms, this is not necessarily the most efficient. As was previously stated, each engine has a maximum efficiency point where horsepower and torque are maximized. Just a guess, but it's probably best just to cruise somewhere between 2000 and 3000 rpm.

sm