The 608ZZ shielded bearing is needed for the X and Y axis idlers. You can use one on each axis with fender washers as called for in the standard Prusa build. Although for a smoother idler we recommend using two bearings on each idler without fender washers. This removes the friction caused by the axis belt rubbing on the fender washer. You can see the use of the stacked bearings on the MendelMax build here.