If you've spun your singles and then immediately turned around and plied, it's very possible your yarn is balanced, and the only reason to wash it would be to remove any oils, dirt or dust introduced while spinning.
I usually leave my singles on the bobbin at least overnight, which means when I ply a balanced yarn, it "acts" overplied, since the singles' twist set a bit while it rested on the bobbins. So, a soak in a warm sink will let everything relax back into the balanced state and I should get a balanced skein that I can simply hang to dry without weights.
That said, I also like to spin singles. I wash them mainly to clean them, and then while they are drying, I hang them with a soup can (a big, 14 oz. lentil soup can :-) ) laced through the bottom loops. If I've done a really_good job of minimizing twist in the singles, then a simple handtowel through the bottom loops does the job.
If my plied skein isn't balanced, I'll weight it with as little as I can, so as not to remove the "bounce" from the yarn -- so I try a hand-towel, and only move up to a soup can if that doesn't work.
So, I always wash the skein and hang it, and put weight on it only if it's not behaving once it's had its warm bath.
That said, I've been spinning crazy yarn lately, that won't take a wash, like the "loop" yarn shown here.
For that one, I put it tightly around a niddy noddy and left it for a MONTH. Whew. Patience required for that one, and a "spare" niddy noddy. The twist is set, it's pretty, and it's art -- since I can't wash it! LOL. (washing would make the loops curl up on themselves, not the intended effect!)
(based on a post by me on Spin-List)