I mostly like to use backings that contain colors from the top. Sometimes, it will be something really bold, other times, not so much. I only buy for projects so I will usually purchase the whole kit & kaboodle at the same time. Typically I will get binding fabric that will coordinate with both top and back.
Thread...I'm a hand quilter but the principle is the same...depends on how much I want my quilting to show - either front or back.