Many in our industry feel that having a customer sign a waiver could give the wrong impression about the quality of their work and think that it is a bad idea. Glass is unpredictable. Without knowing the history of the windshield and how it was installed leaves you open to too much risk. Yes, the chances are rare that this will occur, but it can and will happen at some point in your business career. By taking a moment to explain what you are doing and the reasons for the waiver, you will look like a hero to your customer when all goes well.
Heroes get referrals.
I understand that X Glass will be repairing the damaged area of my windshield. During the windshield repair process, I understand that there is a chance of the damaged area spreading and becoming non-repairable. If this occurs, I will not hold X Glass liable for any damage.