Is it true that too much heat can cause a windshield to crack and break?

Yes, this is true. You can avoid this by keeping your heating gun mobile, and not keeping it in one location for too long. In addition, you might want to look over the windshield for small, pre-existing “spiderweb” cracks before you begin work. If you spot them, you should notify your customer. A soak rope also helps prevent breaks and cracks.