So maximum power can be developed by keeping the shoulder angle such that the arm is straight out from the side of the body.
But in addition, the arm bone at this angle is much less likely to put stress on the labrum and rotator cuff muscles at a 90 degree angle.
So anyone with a history of labrum and rotator cuff problems should be especially careful to maintain this angle into contact, not just pros like in the picture of Roddick above.
And to do this, they have to "drop the left shoulder" just as "TennisOne.com Editor Jim McLennan explains some serving techniques to generate power and protect your shoulder." http://www.youtube.com/watch?v=lTRvxaBMh8s
Do as Jim McLennan says and "drop the left shoulder" and your serve at contact will look like Roddick's above, and like the following pros: