Whether you have been in the restaurant business for many years or you are new to the industry it is always good to verify the work that is being performed in you hood exhaust systems. The last thing you want on your resume is a grease fire due to poor quality cleaning. Yes, the companies cleaning the systems are typically liable for any damage. But YOU run the restaurant/kitchen, so YOU are responsible for hiring a professional company that maintains fire safety compliance. If it feels like you are getting a deal to good to be true, you may want get a second opinion.