In Tom house's book, The Pitching Edge, there is a table listing some of the top pitchers in the game and how far back they take their arm. If straight back is 180 degrees, almost all of the pitchers listed take their arms back more than 180 degrees.
It's hard to pass judgement on this, IMHO. How much of that movement is in the shoulder and scapula? How much in torso rotation? Pitchers need to set up a load between the links in the kinetic chain. What it takes to do that depends on one's flexibility and that differs from pitcher to pitcher.
Unless a pitcher takes his arm back an extreme amount, I don't normally worry about it.