As your typical pet home, i can care less what the guarantee says. I'm not returning the puppy because of hip or any other genetic issues and I probably wouldn't want another one from that breeder if the first one did have issues. I go with a breeder and the dogs I'm comfortable with and hope I made the right decision. The "guarantee" makes the buyer feel all warm and fuzzy inside and that's about all they're good for. At least in my opinion.
In a real life working home where the dog has a specific job, that's a different story. That's where a guarantee actually has some importance.