Okaaaay...good news is that what you have been doing is not 'terrible'. The reason that a person should measure the width or length thru the center of the quilt top, is that sometimes the edges can stretch, that's all. And if those edges have stretched, then the borders will actually be longer than the center of the quilt. When it has been quilted, bound and put on a bed, it could look like a skirt that has been 'sat out'.
So, long story short, measuring thru the center of the quilt top helps to keep the width and length even along the whole sides.