It's also important to know what kind of stretch the fabric you're selecting might have. There are basically two kinds of stretch. The fabric might only stretch horizontally (called two-way stretch) or it might stretch both horizontally and vertically (called four-way stretch). This can be determined simply by picking up the fabric and stretching it both ways. These are not usually interchangeable. For example, if I'm making a fitted gown out of a knit fabric (which I just did yesterday), then you probably want a fabric that has only horizontal stretch. After all, you may not want the gown to stretch downward. Horiztonal stretch will allow the gown to cling to the body without hanging ... oddly after a few washes. But a four-way stretch might be better for a slip cover or tight leggings. It all depends on the final product, so test before you buy.
Also pay attention to the fabric content. Knit fabrics made of synthetic fibres tend to have better recovery than natural fibres, so if you're hoping to make something that will keep its shape after many wears, look for a fabric with at least some synthetic content. A gown made entirely of wool, for example, will eventually lose its shape.
The bottom line is you absolutely must know what you're making with a knit fabric before you grab it off the shelf. Stretch factor and fabric content are very important when you decide to work with knit fabrics, so consider these things before you go out fabric shopping.