It used to be a cardinal rule never to have knots in your work. You should always start a new skein or ball of yarn on the edge so that you don’t get a bump in your work. I think this is good advice which I usually adhere to. It does waste yarn, but the end effect, of having a smooth piece of work, is worth it.
But now I am rethinking this a bit. One of my WIP’s is a blanket knitted on really large needles, and using really chunky yarn, but the skeins only have 27 yards on them – that does not go far! So I looked up on-line how to tie a knot in my yarn. Yes, really. There are techniques that ensure a tight knot that will not come undone, as well as not being a huge bump in your work. I am going to use this in my ‘giant’ work because it seems a more efficient use of those short skeins (I would waste yards and yards if I had to start a new skein when I could not complete the row).
For baby items though, I would definitely skip the mid-row knotting.
Rules are really guidelines only, and sometimes are meant to be broken.