Nope - you are wrong nick - you decided as soon as your first turn starts and your roll the dice to see what one you want to come in. That is how it rolls - just picked up the codex and read it.

"At the beginning of your first turn, divide the army into two groups that must include, as much as possible, the same number of units. Then you must face the unfathomable judgement of the Gods of Chaos, and nominate which of the two groups you wish to make a "Daemonic Assault" at the beginning of the battle. Pick one of the two groups and pray to the Dark Gods by rolling a d6. On a 3-6 the Powers of Chaos agree your choice, but on a 1-2 they choose the other group."

So yeah - you don't decided until you start our actual turn. Fun times indeed.