Neither, do it a little out of the saddle but not over the bars, just unweight the saddle.

At the sprints we were doing last night, we also ended up doing skids since the light misty rain had left a nice slick surface. teadoggg ended up doing 180 skids, very nice.

Skidding, to me, as others have posted above is more of a style/show thing or In Case Of™. I prefer to pick my lines and gauge my route as opposed to trackstanding, skidding or any of that. Plus I've spent some good money on 4 tires in the last week and I'd like my current set to last a while too.