There's no 1 answer, except that it doesn't really matter to the bearing -- profile bearings normally have the same rated load in any direction.
The rail mounting bolts are slightly happier if the rail is loaded in compression than tension. But a router is unlikely to stress a 30mm rail.
The overall configuration design does matter: rail spacing, moment arms to the cutter, distance between the 2 blocks on a rail, aspect ratio of 4 block pattern, and especially the ease of aligning 2 rails parallel to each other. So mount them however to best achieve those objectives and manufacturability of the machine.