The rule(s) I was taught to follow were the same as the ones Sheldon wrote as quoted in post #7 . Basically you made them as short as possible while still being able to fully turn the bars in both directions until they touch the top tube. I'd usually give them an extra inch or so beyond this, to allow some trimming in the future as the ends got mangled. This method is/was possibly influenced by Eddy Merckx, who was famous for his extremely short cable housing. Also, unless there's very good reason to do otherwise, cables should be behind the bars. Generally speaking, you just want the smoothest most logical and shortest path from lever to brake.

The cables on my personal bikes almost never cross, because I've always hooked them up Italian style - right hand front brake.