It depends on the look you want. If you match the thread to the fabric, it does hide mistakes better and gives you a textured look. But, it is hard to see while quilting. Contrasting thread is easier to see, and the quilting design will be more pronounced, but mistakes will be more noticeable. Many people quilt with an off-white or cream color I think. My favorites for the top are variegated threads.
I tend to match the bobbin thread to the back. But, I'm doing much better with my quilting, so I'm not as afraid of the mistakes in stitching showing as I used to be.