as a judge i don't look at choice of backing and those with whom I have judged do not either. The backing is looked at if smooth etc. I think you are trying to second guess the judging process. The comment above is correct about judges at county fairs. A certified judge will not give weight to backing fabric. Others may which enters the idea of personal preference which should not be part of the process.
Even those quilts that are heavily embelleshed on the back are not (in my experience) given consideration even tho it upsets the quiltmaker.
Your customer has chosen the backing. Use it. I don't believe it is your responsibility to question it. What if it is a case where the judge did make a negative comment, then it might be considered your fault that it was changed.