It really depends on the colors or theme of the quilt top I am making simply because colors of the backing fabric are limiting and sometimes the theme or the print just does not want to blend with the top. Another thing that helps me decide is if i have a collection of coordinating fabric to do the top and i know i have enough yardage of one that can be pieced for the backing, i prefer to do that even if i can find backing color to match my top, I'd rather use the same collection.