The use of sheets for backing is sort of like pre wash or not for fabrics. There are many pros and cons for both sides of the issue. And, it depends on what you are using for batting, the purpose of the quilt, how it is going to be sandwiched (tied, machine quilted, hand quilted) and what the top fabrics are ... so many variables ... no hard and fast rule. Use what works for you and be happy with it.
It is all good!