lay a piece of the fabric you would use for border dowm beside the top, the width you would like to use. Stand back take a picture , look at it and see how you feel.also make the picture black and white then look at it. Does it flow smoothly or does it look like you wanted to make the quilt bigger?I have tried this many times and it really helps.I also do this when I pick fabric for a quilt top.Beleive me a fabric that doesn't belong will let you know, or a quilt that doesn't need a border will say so.