the main reason that people try to avoid using different color thread on front and back is that if you have a tension issue, it will show because of the two different colors. If I'm working on a quilt where there is a strong value difference between the color on the top and the color on the back, then I will use two different color threads when quilting. When using two different color threads, I take a lot of time checking both front and back to make sure I have no tension issues.
Rob