I do agree that you've got to get your hips to where your back is straight. That may be down, back, or a combination of both. You've got to see where your body finds that balance. What you don't want to do is overcorrect and go into some squat-lift variation. You need to keep your butt high enough to keep the primary movers posterior. Bringing your butt down further may put it more quad dominant. This is part of why pulling yourself down helps so much, at least for me.
So here's my long and drawn out answer. I'm a big guy (355-lbs). When I warm up for deadlift, I RDL until I've got at least three plates on because pulling myself down will pull the weight off the floor. By pulling myself down, my butt stays a bit further back and higher than if I just tried to squat down and grab the weight. I'll try to post a link later on, but there's a video on YouTube of Brian Shaw going through this cue. You're basically leveraging body weight to almost begin the pull so that when you pull, you pull back and you're at a mechanical advantage.