Depending on what you will do with the yarn, singles are fine. One of the reasons to ply is to balance over spun yarn which will make your knitted stitches 'twisted'.
There are various methods to set the twist after spinning. I do soak in a soapy warm water, drain and soak with clear water. Sometimes, I'll add a squirt of hair conditioner.
After squeezing out the water, I'll 'thwack' it against the counter. This helps full the yarn and sets the twist.
I hang to dry unweighted. Some folks will weight their yarns, but as soon as the wool gets wet, it will resume it's unweighted structure.