Depends on if the wheels are hubcentric or not. Most aftermarket wheels that will fit an MN12 are not. If you're using OEM Mustang wheels after a hub swap, you probably won't need them, but with aftermarket wheels you likely still need to run them ... have to measure the bore of the wheels, or ask the salesperson when you're ordering them.
Most aftermarket wheels are made with a standard bore that accomodates many cars. The hub centric ring is usually used in all those applications to ensure concentricity.
As an example all Borbet 16" wheels use a 72.6mm bore from what I found out.
Long term tire wear and balance is best with the rings.