Many factors can affect your embroidery stitching. Machine tension, proper hooping tension, needles, the threads you choose, stitch count, and density are some of the major players. With all of these variables, it is important to choose a backing for your work that can provide the foundation needed for a solid, stable embroidered design. The entire purpose of using a backing is to provide a secure base under the fabric for your stitches to keep your image from becoming distorted and to help hold up after washing.
There are two main categories of non-woven backing: tear away and cutaway. As its name implies, a tear away backing is easily torn away after your design is complete. It is best used on more stable fabrics such as denim, terry cloth or on caps. Cutaway backings are normally more stable and softer than tear-aways, with the excess amount being removed by cutting around the finished design. Because cutaway backing provides more stability, it is often used for knits or other fabrics that need extra support.
Numerous styles and weights of backing are available and they are becoming more and more specialized. Choosing the style that best suits your needs may take a little experimentation. As a rule of thumb, keeping a mid-weight tear-away and a soft, but stable cutaway on hand will allow you to tackle most any design. It is important to find a tear-away that will tear cleanly away while still holding your stitches and a cutaway backing that will not stretch too much in any direction. Having too much stretch can cause puckering or distortion of your design once your garment is out of the hoop. This distortion can also happen if you have to pull too hard to remove your excess tear-away.
There are no set-in-stone rules. Specific backing decisions are up to the individual embroiderer. Not everything will work for everyone. Your style of stitching can influence your choice in backing as much as the fabric or stitch count can. Have fun and experiment. Sew out several designs with different weights of backing on various fabrics to see the effect on the finished product. Before long you will be able to see what works the best for you.