I also like to piece my backs, usually just a strip but to match the front.

WHO said is is not ok to piece the backing on both non judged as well as judged quilts. Lots of ribbons hanging on judged quilts with pieced backing.
It is simply a personal opinion.

I sometimes buy fabric to match a quilt. It depends on the quilt. Mostly I buy the wide fabric that matches and I like the look. I also pieced the backing and for certain quilts it fits. For me to buy 5 to 6 yards of fabric for the backing is a waste. The wide fabric is reasonable and looks fine. I have a few quilts I used flannel for backing(these quilts are used on the couch or bed.)

I'm one of those who wants the WHOLE quilt to look coordinated. I have had to sew two pieces together to make it wide enough, but it's the same fabric, and typically matches some of the fabric on the front.

When viewing Trunk Shows at my Guild, I'm always disappointed when I see the back is just random squares and rectangles of all sorts of non-coordinating fabric. I make mostly lap quilts, so the backs and fronts are both seen, and I want the continuity from one to the other.

Mine are mostly for charity, and I want the recipient to know that I took the time and trouble to make it "nice" for them.

I use sheets too Hazeljane:) I frequently hit the Goodwill and find some lovely sheets. Not too tightly woven, not percale. I keep a safety pin in my purse to see how easily the pin goes through. If I get any resistance I pass on that sheet. I finished a quilt for my daughter this winter and she says the sheet I used is the softest thing ever.

i try to match or compliment the front of my quilt with my backing i guess thats the way i started doing it and have done it ever since

If I make tablerunners, then I pick a fabric for backing that's in the front. The reason is however your mood is you can turn the tablerunner. But for a wallhanging I don't care I use whatever I have. Often I bought 4 to 5 yards from 2$ fabric in a small print from Walmart. I also buy marked down fabric from JoAnn for backing.

I like to back my quilts with good sheets. They work really well.

I made a quilt using all my kids old cord bell bottom pants and found a striped sheet that made a perfect coorodianting back.

I use sheets for childrens quilts, they need to wash well after all, and I do not hand quilt. I have used Ikea fabrics because the designs are very different.I love buying kits from USA because a coordinating fabric for the back is usually included. All others I do try to match them in but if the need arises I do piece with fabric from the front. Barbara Australia