Part of the time it's necessity. What is the size of your pieced top & how big do you want it to be? I really think your best bet is to go to the pictures section of this board and look at the quilts posted there and take a good look at the borders. You'll probably find that you are drawn to wider (or narrower) borders etc. It's a matter of personal preference. A common practice (but not a standard) is to start with a narrow solid border right next to the piecing to frame it. One thing to keep in mind is to square up the quilt then measure it in the center and cut your border to that size. Don't just cut a big long strip and sew it on & cut off the excess! This way your quilt will keep in shape.