It's easy to put the most importance on your quilt top. That's what you have worked on for so long. But do not discount the importance of your backing fabric. Think of it as the foundation of your house. Without a good foundation your house can crumble. The issues I see with backing fabric include the following items:
1. Is it square?
This really matters to a longarm quilter, especially if your backing fabric has lines in it's pattern. I try to align the top of the quilt design with the backing fabric pattern, AND the quilt pattern. If the backing fabric is not square it can become impossible to align all three of these things. A plain backing fabric, or one without lines can help eliminate these alignment issues.
2. Are the top and bottom edges cut straight?
The first thing I load onto the quilting machine is the backing fabric. I pin the top and the bottom edges to leaders so the quilt can be rolled from top to bottom. If the top and bottom edges of the backing fabric are not cut straight (and square with the sides) it can be very difficult to pin the backing fabric on straight. This can cause ripples in the backing fabric which can result in making pleats on the back of your quilt. Not good! It's best if I can pin the selvage edges of the backing fabric to the leaders, but you have to make sure the backing fabric is at least 8 inches wider and 8 inches longer (4 inches all the way around) than your quilt top. The backing fabric has to be larger than the quilt top in order to be loaded on the machine frame correctly.
3. If the backing is pieced, where will the seam fall in relation to the quilt top?
Sometimes I get a backing fabric that has been pieced and I do not know how the customer wants the seam to align with the top of the quilt. Should it run horizontal or vertical? Does it need to be centered? This is why I like it when people pin a note to the backing fabric telling me which end is the top and which side will be facing out (if it is not obvious). Does the backing fabric design/pattern need to be aligned when pieced together? In other words, do the patterns need to match at the seam? If so, you will probably need more backing fabric than you think. This is something to consider when purchasing your backing fabric and when planning how you piece it together.
4. Will the thread color for the quilt top look good on the backing fabric?
Sometimes a thread color looks great on the top and can clash on the back. I do not recommend using different thread colors on the top and the back. The reason is that it is possible for the bottom thread color to be seen on the top and visa versa. If you use the same thread color on the top and the back this issue is eliminated
5. What backing fabric suits the quilt AND the quilt pattern (cotton, minky, flannel, wool, muslin, linen, etc.)?
If you use a thick pile backing fabric (like minky), be aware that the fibers from the back can be pulled up and can be seen on the top. I call them fuzzies or tufts. Below is a picture of fuzzies or tufts. For that reason it is recommended to always choose a thread color that matches the color of the minky type fabric so that the effect of the fibers being pulled up will be minimized by blending in with the thread color. Also, a less dense quilt pattern will work best with a thick pile backing fabric and will help to lessen the fiber pull-up effect. Minky is a very soft fabric and can look great when quilted. I recommend a short pile type of minky.
This is an extreme example of using a long pile minky backing fabric where the fibers from the minky are being pulled up to the top of the quilt. Using a short pile minky is advisable.