From what I've read around if you tightened the lug nuts while the wheel is still raised off the ground then centering rings may not be necessary. The lugnuts will force the wheel to center on the hub without load. Make sure to tighten progressively (I believe OEM torque spec is 72.5ft/lbs) in a star pattern to aid in centering. Make sure to re-torque the lug nuts after about 100 miles of driving.
If the lug nuts are tightened with the wheel resting on the ground without a centering ring then the wheel may not be forced correctly into proper position. This will create vibrations and other negative side effects while driving.