I think alot of that has to do with your exercise experience. If you're a raw beginner (let's say, 12 months or less of consistent training), you'd probably benefit from a more thorough and specific warm-up. If you've got some experience under your belt, you can get away with a general warm-up, and then diving into the meat of the workout.
Lots of people have lots of different ideas, of course. I've heard anywhere from "1 warm-up set per 100 pounds of working weight" to "no warm-ups, they're inefficient."
I'd recommend a general warm-up, something to get your body feeling "ready" for exercise. Anywhere from 2-10 minutes of moving, playing, loosening up, calisthenics, whatever. As far as specific warm-ups, that depends alot on the sets and reps of the workout. If you're doing 5x5, I'd only take 1 or 2 sets of 3-5, to get the motion down. If you're doing 3x12, I might not take any warm-up.