mconlonx's statement may incorporate a large margin of error. If you sample actual bikes in the field, it's probably closer to 99.9% of them are within +/-0.05mm parallel and don't need facing.
The benefits of going from 99.9% to 99.99% of perfect is not going to be noticeable for the majority of bike-parts consumers.
Originally Posted by
Burton
Suggest you write Chris King, Shimano and RaceFace etc and let them know how much more experience you have then then the engineers they hire to make those decisions.
Well, not so much engineers as the corporate attourneys and their CYA policies. Very few shops have the jigs and tools to accurately measure the squareness and parallelism on the faces of BB-shells and head-tubes (unless they have a resident framebuilder working in the back room). Not to mention the demographics and middle of the bell-curve of bike-shop employees. Very doubtful there's many MEs in the mix. So the safe thing to write in your installation-manuals is to say,
"Always face BB-shells and headtubes prior to installing our product.".