I think that rolling the D20 to determine sides (if there is a dispute) is the standard procedure. As for physically placing armies, according to the SotM rulebook page 7:
Quote:
If you have already chosen (or brought along) your Army, place your cards in front of you. Then players roll to see who places his Army on the battlefield first. Re-roll ties
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I'm not sure if it is entirely necessary to go that far but I could be wrong. This step may be required for these types of maps. But I would agree that the D20 NEEDS to be rolled to determine who is playing where.