For singles yarn, you don't want to put as much twist in as you would for the singles you plan to ply. I did a really loose single that knitted up very well, but I know Brown Sheep yarn has a fair bit of twist in it (If you have any Brown Sheep yarn, get a piece of it, put it in warm water and watch what happens...it will twist back on itself a fair bit.)
How much twist is enough for singles? Just enough to hold it together, or alternatively, when you hold the just-twisted length loosely, it only tries to twist back on itself loosely, not enough to be a decent 2-ply.
(from a post by me on spindlers, 11/12/2001)