Million dollar question!
I can only share my experience.
In my case my trani suddenly started to grind. Thought it was clutch adjustment. Then later same day it was impossible to put it into any gear. Cause was a cracked finger
So this winter I dropped the engine again and did what I thought was some preventive maintenance on my perfectly shifting never before opened 250k trani. Surprise- the cages for all 5 gear needle bearing were cracked but had not fallen apart, which would have resulted at best in progressively more difficult shifting. Third gear synchro and 1st/2nd gear bits could be considered as "while you are in there" maintenance. Rest was all within spec even after such high mileage. Bearings are at top right (four in bags)
