In my "former life" I worked in a knit shop. We discouraged knots. The only time I use them is at the beginning or end of a row. I usually just knit 2-3 stitches holding the tail of the old yarn and the beginning of the new together and leave about 4 inches on each to weave in. Never had a problem with stitches coming undone and it usually doesn't show in your knitting. Course it depends on your pattern, too.