Well, If you're playing ITC, then you choose your warlord trait 'pre battle'. Anything which happens simultaniously, you can choose the order of, and therefore would be able to roll for any strategem pre battle onwards. Easy.
for out of the book..
Looking at the missions rules, it talks about 'battle length', which is when you roll to see if the game ends or not. To me, this would give me the assumption that the battle starts the beginning of Turn 1 and goes until the end of turn 5,6, or 7 depending on how the dice rolls end up.
Therefore, I'd say that anything which is 'pre battle' would take place any time between mustering your force, and ending with the roll to seize the initiative.