It depends on the pose. Usually if the model is standing, I can measure the figure in terms of his/her/their heads and establish proportions of the figure first before laying down the rough draft of the figure. Sitting poses are easier since there are less heads to count. Measuring is a bit tedious, but it's one way to set up your figure drawing. For more complicated poses like the ones above, after blocking out the poses, I usually do a guesstimate on how the head looks, but, before putting any details, I think looking at the shape and size of the head in relation to the other body parts of the figure helps me see how the head needs to be fixed or edited so that the pose looks accurate to the whole model.
Also, Iin Michael Hampton's book, at least for figures who are 8 heads tall, the pelvis is approximately one head, while the rib cage is approximately 1.5 heads. So that's another guideline to follow if you wanted to check to see if your head looks right on the figure.