After assembling hundreds of bikes over the years, I have refined my approach to how a bike should be cabled. I have two goals when running cables on a bike.
1. Create the cleanest, most efficient route with very easy loops rather than harsh bends.
2. Keep the housing off the paint.
First, cut the front sections of housing so that there is one arc around the head tube. A lot of riders will try to have the rear derailleur housing on the right side of the head tube. All this does is force the housing onto the head tube and the paint. And it creates another bend in the housing. A setup using SRAM, Campy or the new Shimano levers will look the same as the below. Riders using the older Shimano setup will have even easier bends coming from the STI levers.
The rear brake enters the left slot, the rear derailleur enters the middle slot and the front derailleur enters the right slot. Then, cross the derailleur cables. This allows the rear derailleur cable to exit from the right slot. On bikes with down tube routing (this includes road bikes of course), the derailleur cables will cross under the down tube. On some frames, the layout of the braze-ons will not allow this. In that case, you will have to run your cables the more common way.
Create your housing loops at the seat cluster such that the housing rest on the seatpost rather than the paint on the frame. And better yet, you may put a wrap of tape on your post for the correct saddle height and then have the housing rest against the tape. Nothing gets rubbed by the housing except the tape. Also remember to run the housing in such a way that you can get to your seat post binder bolt.
And that’s it! You are left with very tidy housing runs and none of the housings are touching your frame.