Depends how the cable lines up with the next housing stop/segment of housing. The preference is a straight line from one stop to the next. The exception to this would be if you are trying to avoid a sharp housing radius that may cause friction...an examle of this is routing STI/Ergo shifter housing around to the opposite side of the bike's cable stop and then crossing the cables under the d-tube where they are realligned as they are routed through the cable guide under the bottom brocket.

-j