I think just do what you want and/or can afford. It's all good. I will piece a back if I need to but I prefer not too. Not because I like the look of a solid type backing but mainly because by the time I get done piecing the front, I just want to get the backing on and get her done. If I have bought special fabric for the front I will try and use a good quality backing for it. If it is a quilt I pieced out of scraps I will use what I have. I made 7 lap quilts out of scrap flannel but did not have enough flannel to back them so I bought a cheaper flannel at one of the box stores on sale, actually washed it in hot water twice because I could see that it had a looser weave. The hot washing helped shrink it all up and then I put it on the back. I can still tell the quality is not as good as the scraps on the front but it works and I have since washed it several times, It is soft and cuddly will all the washing and use. I still did not have enough flannel for backing so I did use a brushed homespun on the back of two of them. They are just fine although I really do think they will wear out sooner than the all flannel ones but they were made of scraps in the first place. I have been given sheets a couple of times and did not have any problems with them on my long arm. The quality of sheets really varies widely as does the content so I really do try and stay away from them. I keep my eye out for the sales rack and if there is a fabric of enough qualtity that I think would make a fine quilt back, I will get it even if I don't have a specific quilt top in mind. I do stay away though from unusual fabrics or directional ones that would be hard to match in a long seam. I still have some of those but I have found several of those bargin backings that have found their quilt top and sometimes, I really don't think I could have found a better backing if I had bought it at full price specifically for a top.