Damaged squeegee rubber is the number one cause of streaks and smears on window glass regardless of how skilled you are as a professional window cleaner. To avoid this, pay careful attention as to how often you are replacing the rubber blade.The cuts on the surface of the blade and the rounded edge as a result of normal wear and tear affect the efficiency of the job.
Both sides of the rubber are usable. Try flipping it when one side gets totally worn out, so you can maximize your investment. It is also an option that you can remove one of the two end clips, so the rubber can lay flat on the surface.
In the case of torn rubber ends, simply cut the torn part and reinsert it to a shorter squeegee channel.