Apart from front brake (which run down direct to v-brake or disc brake caliper), rear brake, FD and RD require routing either via top tube, bottom tube or both (brake on top tube, while FD & RD on bottom tube).
"Across the head tube" route
FD which is on the left side, the cable runs across the head tube to the right side before tied to top (or bottom) tube. RD on the opposite, runs from right side across the head tube to the left then tied to top (bottom) tube. For rear brake, since I fit on left side, runs across the head tube to the right side of the bike.
"Same side" route
All cables run on same side of the bike, i.e. no cable cross the head tube.
Which is the best?
While there is no hard rules on how one should route the cable, I prefer "across the head tube" cable routing. I have 2 reasons of doing so: -
1. to reduce cable casing rub
Cable casing doesn't rubs on the head tube thus save the head tube's paintwork.
2. smoother cable movement
Cable routing has lesser degree bend in between 2 fixed position i.e. the lever's barrel adjuster and cable clamp on the top/bottom tube)
On my Spesh, FD & RD run on bottom tube. By default, casing stopper (I will lookup the actual term use for this little thing below my bottom tube) for FD on left, RD on the right. I overcome the situation by crossing the casing underneath the bottom tube (see photo below).
To all my muslim friends, Happy New Year!