I measure the quilt before adding the borders. Then the completed top is measured to determine how much backing is needed. I've noticed some shrinkage after the quilting is done. Another measurement is taken for the binding. If it is a lap quilt or a coverlet, it is washed in cold water, gentle cycle. There is usually a bit more shrinkage.
The only time I am concerned about the actual measurement is when it is a quilt of valor since there are minimum and maximum requirements to keep in mind.