In which phase the hidden deployment troopers are set up? After everyone have deployed or during the normal placing respecting initiative?
You can certainly place it during the main deployment of your army (with any other deployables that they also have - some HD units have minelayer, for example. ). You can also do it as the reserve deployment (or as one of them, if you have an ability that allows multiple "reserve" models.). It will depend a lot on whom is deploying first, too. If you deploy first, then holding it for your reserve can work, because that way you would get to see the bulk of your opponent's force deployed first. It's a situational thing, and also dependent on which faction your opponent brings.
