Thanks again for letting me ask my questions here. Do I stretch yarn that I've washed and rinsed to set it or do I not need to stretch it while it dries? If I've set single-ply yarn, can I later ply it with other yarn and reset the two-ply?

The only reason to stretch or dry yarn with a weight, is if you are going to use it for the warp threads in weaving. It allows the warp threads to be smooth and even and already stretched out for setting the loom, making it an easier job.

If you are using the yarn for knitting or crocheting, don't use the weight. Stretching a yarn this way that you'll use for knitting may very well throw your gauge out the window. When the finished garment is washed, the yarn may un-stretch and the resulting garment could be smaller than you intended.

If you're washing a single that is over twisted and needs to be straightened somewhat, just leave the skein very wet and hang it over your bathtub to drip. This will help remove straighten the yarn without stretching it out.

And sure, you can always ply the single later one.