Liz, you used a very reasonable approach. In my opinion, as you continue to honor the readiness of your students and coach them to always give their best, they will value it and probably not take advantage of the situation. If you detect anyone taking advantage, a private conversation might be in order. They will soon see that the purpose of the quiz is to assess their ability to master the content/skills and not to 'catch' them for not studying. The quiz grade certainly has more meaning and is a true reflection of a student's mastery.