If you have enough of them, I would put some out now for deliberate exposure to the chilling temps of winter.

For the rest, I would be tempted to go by the directions in this interesting article on just that subject:
http://trees.martinsvillenj.com/sequoiaseeds.htm

The Giant Sequoia grows naturally in the very coldest areas of the western continental United States, where temperatures routinely drop below -32F.

Sue