Some dogs are very happy to guide their blind human, others would feel stressed out by such a task.
From what I've seen, the schools seem to be very responsive to this type of issue. If a guide dog user reports that the dog is showing signs of stress while doing their job, the school will normally send someone to evaluate in the user's home location, and if there is an issue, they will either career-change the dog if possible, or retire them, and they will become a pet for a loving home. If it's a good school, they will be concerned about their dogs' welfare, as well as the safety of the users, and will take whatever action is required to maximize both.