Depth of field at 1.2 and even 1.4 is paper thin if doing a head & shoulders portrait. I wouldn't rely on the autofocus that much in those situations and manual focus for the eyes. Even recomposing slightly will move the focus right off.
I wouldn't want depth of field any thinner than my sigma 1.4. Bokeh is dreamy gorgeous.
I'm doing some fairly deep water speculation here, since my shallow DoF photography experience is not as good as landscape photography (which is my main interest). Anyway - with my limited experience so far it seems to me that the difference between f/1.2 and say f/2.0 concerning focus placement is not big at all - in both cases you need to nail right it within say 5mm. The thing is that already at f/2 the DoF is short enough for you to see on exactly where in the face the focal plane sits so you already have the problem, and it does not get much worse at f/1.2. Am I right or wrong about this? Someone more experienced could comment.
Concerning focus-and-recompose, I've done some calculations and the only time you get issues is when you change your angle really much, which typically only occurs with a wide angle lens. Head-and-shoulder portraits is no problem, with a typical 85mm headshot you change the distance with ~1.6mm by recomposing. The longer tele lens you have the less of an angle is (typically) required to recompose and the lesser is the problem. I don't think 85mm in typical use will have much of this problem.
Still, Hasselblad has a feature for this, "True Focus", so the problem exists in some conditions, here's an illustration I found on the web:http://akelstudio.s3.amazonaws.com/blog/wp-content/uploads/2011/12/true_focus_illust.png
it shows the angle of view of a 24mm lens though, and in that particular example the focal plane will move about 130mm.
So far I have shot short DoF with 70-200/2.8 and 50/1.4 using autofocus central point only and I'm quite pleased with the hit rate, so I think autofocus and recomposing will work.