I always get confused about "Z" and "S" twist. I find it easier to think about clockwise and counterclockwise. (For the record, Z = clockwise, S = counterclockwise. For a variety of physics-induced reasons upon which I will not elaborate, I reverse these constantly. Best to stick with the clock analogy...).
Anyways, from my own experience in knitting both "continental" and "british" styles (aka "picking" and "throwing"), I find that I, personally, need my yarns to be plied counterclockwise. Otherwise, no matter how I knit, the yarn unplies, which is super annoying. This single factor overrides all other considerations for me, so I've not intentionally played with plying direction, and I certainly haven't compared plying direction and stitch definition (like this article explores). Other knitters find that their throwing and picking styles seem to require opposite plying directions, but I don't find this. Since I don't crochet (willingly), I haven't explored if my crocheting style requires a different plying direction from my knitting.
Anyways. For me, I know that my final ply must always be counterclockwise. This means that for regular plied yarns, I spin my singles clockwise and ply counterclockwise.
For standard 4-ply cable yarns, which I do a lot of, there's an extra plying step involved, so my singles must be spun counterclockwise, then plied twice: first clockwise, finally counterclockwise. This is where I've made mistakes - starting off spinning clockwise, and ending up with clockwise-plied cable yarn, which unplies as I knit. Lesson learned!
For 3-ply cable yarn (or crepe yarn), some of the singles must be spun clockwise, and some, counterclockwise. Yes, it gets complicated! I label the bobbin, marking the spin direction, if I'm making this type of yarn, so I don't mix them up!
So the lesson here is that I need to decide in advance what my final yarn is going to be, before starting to spin. It also means that by now, I can spin clockwise or counterclockwise with equal ease, with very similar results in terms of uniformity and twist. I think maybe that's why spinning for socks is such good training! It really has led me into learning about all these techniques and has improved my spinning skillz!