Originally Posted by
79pmooney
+1 Set up the housings for runs that are as clean and straight as possible and also allow the bars to be swung to their stops (bar hitting the top tube or brake caliper the down tube) so that swing doesn't kink the cables. Yes, that swing should never happen. (Nor should derailleur bikes ever see the big-big gear combination. Nor cars bump each other. But nearly everyone checks the big-big when they put on new chains and drives cars with bumpers.)
If two different routings pass those criteria and you like one of them better visually, go for it. It's your bike, not ours. I like the visually elegant. But this is the most important system on the bike, possibly by quite a lot.
One possible exception: it can be a bad idea to rout the brake housing under the bars for non-aero brake levers. If the housings are the sag-prone old-school type and the bike gets ridden much, the housings have a tendency to droop as the miles accumulate, resulting in a kink just above the brake lever ferrule. Not necessarily dangerous, but unsightly.
In any event, I don't think I ever saw a bike catalog from the pre-aero-lever era showing cables routed under the bars, so there's that. Doing so while assembling a new bike was verboten in every shop I ever worked in back then.