I keep my head up & attempt to watch the ball from my release until racket contact on my serve. However, when my racket moves upward to contact the ball, it is moving so fast that I do not believe that I've ever really "seen" the racket contact the ball. My head does move forward a bit (& out of the way) some time after the toss so that my serving shoulder can come forward & upward to put my serving arm in the proper position for contacting the ball.
Even tho' the head moves a bit during the service motion, I try to keep my eyes fixated on the ball and expected contact point until contact. I do not pull my head down prior to contact to look at my intended target area as my racket is about to make contact. That action would tend to throw off the swing path of my racket. At that point, it is probably too late to consciously alter your swing while you are looking at the target area or your opponent.
Instead, I prefer to visualize my service placement prior to commencing the service action. With this visualization and the visual sighting of my toss, I let my "kinesthetic memory" subconsciously figure out how to achieve the desired serve.
If you study Federer's serve, his eyes and head do pretty much the same thing that I described for my serve. On the other hand Roddick, pulls his head down earlier and, in doing so, takes his eyes off the ball a bit before contact. However, I do not believe that he does this in order to look at the target area or the receiver. It appears that he does this because it enhances his upper body snap forward for the contact that is very evident for his service motion. I believe that this may be too difficult for most people to accomplish reliably (w/o a detrimental effect on swing path). Most players would probably be better off trying to emulate Fed rather than Roddick in this respect.