Break in
I have run the motors at half speed and or just seated them in with a Brush stone at Full Speed.
(1) My take on this is just use a brush stone,
(2) you may have to try seating them in a few times untill arcing disappers
(3) if there is any excessive arcing after that, this usually indicates a worn Armature.
Only big problems I have run into is from over-tightening the fan nut as this can put extra load on the bearings and cuase over-heating.
Especially critical on Uprite Vacuums as the Shaft will get hot and eventually melt the Rubber belt.
Having said that Half Speed or Full Speed..........I don't think it has any bearing on a proper break in.