There's no hard and fast rule. What matters is the path the cables take after they leave the stop, and which provides the best alignment, and least tangle.
On your bike, running the brake to the middle and the derailleur to the outside would have the derailleur cable remain below the brake cable avoiding the crossing situation you have now. It may not really matter, but might prevent chaffing of the housings where they cross.
BTW- moving the brake over is easy enough, slacken it at the brake by removing the noodle, the same as you would to change a wheel. Then you can slide the housings out of the stops and reposition them to the center.
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