I think all of my videos work abs as part of the lower body workout after the legs. I can't think of any of my videos that work them before the stretch. I don't think it matters when you do them. For example, if you chose to just work abs as your body part, you would obviously do them after you warm up.
Some Firm videos work them through out the workout.
In My humble opinion, I don't think it matters when, as long as you work them! I think it is more important to constantly change your order and ab exercises, as this muscle has an amazing memory.