You don't get to chose.
On turn two you roll for your reserves on a 4+ they are on, if you declared them deep striking, they have to. If they don't roll to come on, then you wait until turn 3 and then its on a 3+, if you fail, turn 4 is 2+ and turn five if they are not on, they come on automatically.
I don't know of anything that has the option of choosing not to roll to come on.
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