On trick is to cross the cables under the downtube by running the right-side shifter cable to the left boss and vice versa. This greatly lessens how sharp you have to make the bend between where the cable housing exits under the tape down to the boss. This might allow you to cut the housing shorter or use a housing that would be too short otherwise.
This also lessens or elminates abrasion of the housing against the side of the head tube.
The cables may touch under the downtube but the angle is so oblique, it causes no problem or wear.
- Mark