Originally Posted by
dsaul
Unless you are using an external bottom bracket, its probably not necessary. A facing tool removes metal, so you don't want to do it more than once on each face or you'll end up with a BB that is too narrow. There is no need to protect the bare metal edge, some grease from the threads will always end up there and rust is not a problem.
...this. An old style, ball bearing, cup and cone BB will pretty much take care of any minimal alignment issues. External bearing BB's not so much. The majority of the bikes you'll run into have been faced once already.