It depends on your medium. A good wool has little hooks along the shaft of its staple so once knit the yarn will cling. Cutting a single stitch will not cause a loss of coherency. In fact, a classic knitting technique for crafting something like a cardigan made with wool involves knitting in the round (making a spiral tube, essentially) and then slicing it open for the button band and sleeves. It's called "steeking".