Lots of creative ideas here. I mostly make kid sized or "throw" sized quilts, and I imagine that the backs will be seen a lot. Lots of tossing to the side and folding up, etc. So for me, it is kind of important to make the back interesting. One of my favorite combinations is a bright pink and orange Kaffe Fassett pieced top, and white with black polka dots back. I also buy wide backing when I find it on sale. And, I was one of the lucky people who was able to get to Hancock's Fabrics before they closed, and bought "rest of the bolt" so I have several pieces of yardage to browse before I go spend some more money.
I sometimes add some of the top colors to the back, but even with my "heavy duty" machine, it sometimes complains if it has to quilt through a bunch of seams. If it's gonna skip stitches, it will do it there.