I would almost never cancel an AP score, and here's why.
AP scores are literally NEVER required for college admission. Students who excel on their AP exams can send them, but admission offices won't wonder where they are if they are not part of a student's application, and students will not be penalized for not sending or reporting AP scores.
AP scores ARE used to earn college credit after you enroll at the college you choose in the end. Most colleges will award some credit for scores of 4 or 5; some colleges will award some credit for scores of 3. No colleges reward credit for scores of 1 or 2, but there is no downside to having these scores show up on the AP report when you send your AP scores to the college at which you enroll.
If you cancel an AP score, you will never know what score you receive! So I really can't think of a situation in which I would cancel an AP score.