Tom Fletcher (Nottingham Analogue) used to do that on the Mentor Reference models when the numbers of turntables he made were small, before his company took off. He used to lap each bearing in by hand, normally whilst watching Coronation Street. This was done so the two parts of the bearing were perfectly matched. If you are making in numbers, then tolerances are generally eased to meet production runs where there will always be greater margins. SME did this with the first runs of the SME V tonearm. SME soon realised that the super close tolerance bearings didn't always fit, or they were too tight, so SME ended up using the slight lower spec bearings (still very high) that were used in the SME IV. After time the SME IV and V ended up using the same bearings.