I think, the tilt of the head is to express attention and obedience to Christ, Who is often the central figure, or to other person of God, as in the Pentecost icon.
That makes sense, thank you. I was really noticing the particular angle of the tilt in many icons I have been showed. It just looks odd to me for some reason. :) But perhaps many of those I have seen were from the same general time period, and that was simply the artistic style of the time.