In most cases, the range of motion of an exercise is dictated by the range of motion of the muscle/joints involved. If you are performing an exercise that you feel goes beyond your natural range of motion it is advised that you stay within "your" natural range of motion. I am thinking particularly of shoulder exercises. The shoulder has a large range of motion in multiple planes, but the joint can also become unstable or place unnecessary strain on the small muscles of the shoulder in the stretched positions.
I guess what I'm trying to say, is be smart. Think long term joint health first.