I also try to match Terry's clock in my allignment. I find that as I draw the bow, my shoulders tend open up, to point to the left of the target, in the early stages of the draw. It order to get my shoulders perpendicular to the target at full draw, I have to rotate them back into allignment. This feels like I'm pushing with my bow arm, and like I'm moving my left shoulder in towards the bow. I don't know if this is the same thing you mean by your "tuck" but it sounds like the same movement I'm describing above.
I don't know if I should be making this movement during the draw, or if I should just work on keeping my shoulders in allignment from the start to the finish of the draw, and maybe Terry can comment on that.