It's not really needed with modern manufacturing. The component makers list it as a requirement mainly to cover their ass.

Example: A shade tree mechanic fubars a bottom bracket install due to his own incompetence. He calls the distributor to file a warranty claim. The distributor asks if the BB shell was chased and faced (knowing fully that it requires special tooling that the home mechanic most likely does not have). The home mech replies, "Uh, what's that?".

Result: Warranty denied.


Use some common sense (removing excess paint from threaded and press fit areas) and you'll be fine.