Bodybuilding exercises should take a muscle through its longest possible range of motion, with some exceptions. You should take care to stretch out to full extension, and then come all the way back to a position of complete contraction. This is the only way to stimulate the entire muscle and every muscle fibre. Whether you are doing 8, 10, or 12 reps for example, it is assumed that you are completing each rep to its full range of motion.