If you are getting Omniums (or any other external bearing crank for that matter) PLEASE spend the extra few bucks and get the frame faced. I work at a shop and for the longest time I was one of those that thought it was not THAT important to face the frame for external BB cranks. We recently bought a facing tool and all of our cranks are running soother than before. We've even gone as far as calling customers who recently bought external BB cranks and offered to face their cranks for free if they wanted to come by and drop the bike off for a bit.

Sure, the bearings are not going to explode if you don't face your BB shell but you'll extend the longevity of the entire system. I would say that in the case of some high end bikes like my Tarmac which have a machined BB shell it is not necessary but pretty much required to anything else.

I recently purchased an Argon 18 Electron which appeared to have a faced shell. There was no paint on the shell faces and the Aluminum was smooth as if someone had faced it but after a few turns of the facing tool metal began so shave off. The Omniums I installed are the smoothest external BB system I've encountered (next to campy).