Once the tags start to get all raggy, your sewing line doesn't show at all.
Three colors of thread will work for most all of your quilting blocks: Off-white, Taupe, and Black
These colors will blend into most all of the blocks you sew. The color I use most is taupe. By using these 3 colors, you can keep plenty of bobbins filled ahead of time, so you don't have to stop and refill a bobbin in the middle of the project.
The only time I use a specific color is when I am sewing on top of the quilt where color matching is important.