Unless there is a mixing issue, you should use a low back pressure when possible. This approach will minimize the energy consumed in recovering the shot.
Back Pressure: Any additional back pressure will cause the screw to rotate additional turns due to the polymer backflow over the flights of the screw. Using more back pressure than necessary to create a consistent shot can causes a 5-10% increase in the energy consumed during recovery.
Recovery: In most processes, there is more energy consumed in screw rotation than any other aspect of the process. As a result, any reduction in energy consumption during screw recovery will benefit the overall process.