I am off and knitting on my first piece of clothing--a cabled shrug (i.e. a cardigan without all that cardigan to worry about) for which I bought yarn last weekend. I have swatched, gone down a needle size, and swatched again. And look!
I decided that getting hopelessly, irreparably tangled up in a swatch was better than having to rip out part of a sweater, so I pulled out my new cable needle, followed the directions in the pattern, and voila! A perfectly good cable, right down the middle. It's amazing how that works. New skill: check!