It seems largely to depend on what you will do for lighting during the winter. I've started cuttings in Z6 in December, but then the trees are 2-3' high and 2' wide by May. An early indoor start works only if you have good indoor lights and maintain care transitioning the trees to real temperatures and sunlight. I've had cold, wind, and sun damage after moving the cuttings outside too soon (late April) in order to make room under the grow lights.
Late Jan to early Feb seems better to me, but still only if you have decent grow lights. Window light generally doesn't seem strong enough. Without artificial lights, I might wait until March so that the growing can get started indoors but begin in earnest outdoors.