Wikipedia:
Centerbore
The centerbore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Factory wheels have a centerbore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct centerbore to the car they will be mounted on are known as hubcentric. Hubcentric wheels take the stress off the lug nuts, reducing the job of the lug nuts to holding the wheel to the car. Wheels that are not hubcentric are known as lugcentric, as the job of centering is done by the lug nuts assuming they are properly torqued down. Centerbore on aftermarket wheels must be greater than or equal to that of the car or the wheel will not physically mount to the car. Many aftermarket wheels come with hubcentric rings that lock into the back of the wheel to center it like a factory wheel, usually made of plastic.
Edit:
Well, seems you're right & I'm wrong.
However, I would never go without the rings for the reason described in
this link.
