Back in the days of unlined cable housing the recommendation was to tape the cable housing from the shifter to the bottom of the bend in the handlebar and then have the housing and cable loop in a graceful curve to the downtube. This was to improve the shifting as at that time running the housing and the cable up to tthe stem resulted in additional curves and therefore more friction. With improved cables and lined housing that doesn't seem to be a prblem since other posters have done it to the stems with no deterioration of shifting.

BTW, some people run the left cable to the right downtube boss and from the right shifter to the left downtube boss is oreder to 1. get a more graceful curve in the housing and 2. so that the housing clears the head tube and doesn't rub the paint.

Cheers