I have round aluminum bars. Both the shifter and brake cables are routed to the bottom front of the bars. The cables fit right at my first finger joint when I'm gripping the top position. That's very comfortable. And I like the smooth curve on the back where my thumbs wrap around.

At the shifter, one of the two cables crosses over the bars just behind the hoods when it's routed to the front of the bars. To avoid a bump there, I filled in at the sides of the cable with a couple of layers of scrap pieces of old inner tube, held with electrical tape. Then there's no bump under the bar tape.

So cable grooves might not be necessary for you.

Carbon assembly paste would help to reduce the necessary torque if you get carbon bars. The paste really works.