I don't stress it at all! As a LAQ, I love the ease of using wide backing with no piecing involved, so I have a lot of wide back in my stash. I buy what is on sale. I have it from very busy prints to wide muslin to tonals. Sometimes I want a flannel back on the quilt. Then I just try to choose some colors or prints that compliment my top. Sometimes I use leftover blocks or scraps from the front and do a pieced back. Sometimes I don't go by color at all but go by theme, like I did a quilt with tigers and dragons quilted on the front. The quilt was black with red yellow and orange in batik fabrics on the front. The backing I chose an asian print with dragons and tigers that was a tan color so the color did not at all go with the top, aside from the dragon and tiger theme. On one recent quilt, I used an inexpensive walmart fleece blanket. In my mind its all good and there are no rules.