I find it to be better to go out in the sun as soon as you plant it, if the tree is still dormant. I agree that it takes some time for the roots to establish themselves in the container, however a little warmth from the sun will go a long way to help. Since the top is dormant, transpiration is at a minimum, and when the tree does break dormancy the leaves will already be acclimated to the amount of sunlight the tree gets. Don't do this during the heat of summer though.
~james