There are no rules. from a functional standpoint you want a nice clean cable run from the handlebar to the fulcrums on your frame.

Otherwise do your own thing. You can tape all the way including over the flat section, end at the flat to round juncture with the cables emerging there or keeping them under the bar with a piece of electrical tape supporting them near the stem.

Whatever works for you is fine, but keep in mind that having the cables leave the bar anywhere outboard from the stem might limit your hand positions. What you might do is experiment, using your old bar tape and some electrical tape to find your favorite arrangement, then if you can pull the cables apart trimming the housing for the best cleanest run to the frame. Afterward you can do a nicer tape job with new tape.

BTW, you might find the flat section too slippery to use as a hand position if you don't tape it, and might end up preferring a classic tape job.