I have run the cables under the tape to the stem before and I will likely do it in the future. But there is noticeable extra friction even with new cables and housing. Also there is some tension in the steering with the cables so short between their exit from the tape and the guides.

You must tape top to bottom--from the brake levers at least. With Bike Ribbon Pro and maybe some others, the edges are tapered so nicely that there is no curling from siding down the ramps.

I try for the fewest smoothest curves in cables. (And of course an elegant flow is strived for.) On my Milwaukee, with the cables run to the top tube, I wrap the cables up into the curve and they head right to the cable guides. This also lets me slide forward in the drops braking without the cables catching my thumbs.

On my Sachs, the cables go to the usual position and I wrap them just past the flats so they gracefully arch into the angle of the downtube.