To go with what Brute said, do what allows you to lift the most weight. I think retracting the scapula is a way to keep the back flat, but I don't know. I certainly round my upper back when deadlifting. I seem to lift more weight that way than keeping my entire back flat.
To drift away from your question, some recent cues I have been using:
1. Hips back and down
2. Drive my legs into the floor like all hell.
It seems to be working. Oh, and those two cues will happen in less than a second.