Heavens! By all means use white thread on white fabric, but if the fabric is a print or a solid color other than white, Try laying out different color threads on your quilt top, then pick something to compliment your top. Do you want the quilting to "pop" - use a darker thread color than the fabric. Matching the thread color to the fabric will let the eye see the quilting texture more than the quilting stitches. Have fun and try different threads. You may even want to use several different colors of thread on your top.