Detail jittering 'jitters' the level-of-detail decisions, analogously to jittering pixel intensities in a bitmap image. With no detail jittering, the decision about when to subdivide a micropolygon edge is based on only a few level-of-detail criteria. It is possible to have some areas of a surface with many polygons that are slightly too large next to other areas where the polygons are much smaller. In an animation, the lines between different level-of-detail zones may be noticeable. With detail jittering, each polygon edge has a random value that affects its level-of-detail criteria, and the result is that polygon sizes are distributed better. Although it's definitely a good thing for animation, I would always recommend that you have it on for stills too. It should have a negligible impact on render times.
Matt
