All my bikes are front/left:
- I can brake safely and indicate a right turn;
- with DTS, I can brake safely and change gear (I only operate both shifters with my right hand anyway);
- the cable enters the front caliper on the right-hand side, so left/front gives a smoother curve which is more efficient; equally, having right/rear makes for a smoother curve as the cable guides are on the left-hand side of the top tube.
On a side note, I also route the gear cables of my Ergo shifters the "long way around", so that the front derailleur cable enters the right-hand down tube cable stop and vice versa, crossing under the down tube to enter the bottom bracket cable guide as normal.
As well as a smoother curve, this avoids cable rub on the head tube.