Just want to drop in my bit of knowledge. I've heard time and again not to stretch before a workout, due to the belief that it elongates the muscle and weakens it. In this case, you'd want to just do a few light warmup sets to go through the motions, get blood flowing, and save the stretching for the end of a session in order to reduce soreness.
Now, I've tried both ways, and I honestly haven't seen any difference in strength or performance when I've stretched. In fact, I feel better going through my squats and deadlifts after stretching.
My suggestion? Walk the treadmill or ride a bike for 3-5 minutes, just to get your heart beating a little faster. After that, do your warmup sets (the lighter ones that require little effort) and between those, stretch lightly.
There might not be scientific evidence behind this, but I don't see much that says it's detrimental, either. I don't feel the need to do such things with bench presses or military presses, but I have to stretch my groin, glutes, quads, and hamstrings before heavy squatting and deadlifting.