In a manual throttle aircraft ( e.g. Enstrom) flying with Nr at the top of the green will use about 1 US gallon per hour more than flying at the bottom of the green for the same airspeed.
Watch out for recovery from low Nr by opening the throttle(if its not already open!). The extra torque may overpower the limited TR thrust. i.e pilot induced LTE. Safer if possible to lower the lever a bit first. This type of LTE is easy to (inadvertantly) demonstrate in an Enstrom and easy to recover if you are on the ball.