Doh!
Those aren't spacers, those are hub-centering rings.
I'm not so sure whether these plastic ones do good enough of a job, or at least help center the wheel, but I have mine on anyways.
I highly suggest you put them on as well, it certainly won't hurt.
Also make sure you torque down your wheels while they are in the air, not on the ground. This is important with lug-centric wheels, otherwise you might be picking up your lug nuts and studs off the ground.
John
