Well, it's generally easier to do it that way.
For one, the tabs are a little longer than they are wide so it's natural to lay the wire on that way.
Second, soldering them perpendicular to the motor will cause the wires to overlap (obviously), thus are more difficult to solder (but not impossible). Also consider that any overlapping wire will have wear on the insulation from the soldered joint beneath it caused by vibration/movement and may eventually cause a short (not good for the ESC). The only other option would be to solder the wires "on-end" to the pads, which can be even more difficult and result in less contact area.
Personally, I'd just solder the wires inline to the motor and just give yourself an extra two inches or so or wire to allow the wires to loop nicely where they need to go. Kinda like how the wires on my Ten-T are arranged:
If that extra looping gets in the way of a-arms or whatever, consider making a bracket and zip-tire the wires to keep them secure.