Do your wiper blades leave streaks when it's raining? Do they "chatter" when there is a very light rain? Or do you see pieces of rubber hanging off of them? Then don't wait until they fall apart and leave scratches across your windshield. More importantly, don't let them put you in an unsafe situation where you don't have a clear view of the road when it's raining!
Maintenance Tip -----
Wiper blades should be changed at least once a year, or when any of the above start to occur.