Mot of the time I just do warm up sets, but every now and again I feel like I need a good stretch. Sometimes before, sometimes after, depends how I feel on the day. I use to do a pre-workout warm up and go straight into work sets but haven't done that for a long time now.
The way I structure my workouts is around a big compound lift first, so I find doing that particular lift with a few light sets warms me up for the other exercises quite good. By doing this I rarely, if ever these days, have to stretch.