As much as I detest working with interfacing, sometimes it's inevitable. Sometimes you need to stiffen up your fabric, or just add a little more stability. Knits, like other fabrics, can often benefit from a little more stability, depending on the project, of course. Unlike other fabrics, however, knits require a very specific interfacing. Woven and nonwoven fusible interfacings are simply too rigid and usually cause the knit fabric to bubble or pucker after fusing. And after fusing is too late to fix it.
Prevent this problem by choosing your interfacing with care. Tricot knit interfacing, available at most fabric stores, is lightweight and stretchy. This makes it less noticeable when fused to knit fabrics. It also won't damage knit fabrics the way other fusible interfacings can. So do yourself a favor and choose the right interfacing based on the fabric you're using. Don't just grab whatever happens to be on the shelf.