You should always use cut away, on flats, when possible. With cut away backing you receive the most stability. As you embroider an item, you puncture the product with thousands of holes. Backing is used to hold the item stable as you destroy the fabric. If you are using tear away, it does not have strong stability and can not hold the fabric as well as cut away stabilizer.
If your end products is wavy, puckered or misaligned it may not have enough backing or it have the wrong backing.
The more stitches the item has the more backing it needs.