I also think that mid-upper back strength can alleviate a lot of issues with all 3 big lifts. I was trying to help some kids bench better the other day, and they couldn't arch for sh!t. They also couldn't hold their shoulder blades back. This, of course, has nothing to do with mirror muscles and everything to do with the fact that these guys had never rowed a thing in their lives. If I thought they'd actually listen to me I'd tell them to stop benching anything other than the bar for 6 months and do rows, pull ups and dips 2x a week. They'd all add 50lb to their bench.
As far as the dead lift, I know that when I'm overly protective of my lower back then I just can't get any real weights off the floor. I DON'T round my back, but I also don't force an arch. If your back is as strong as it should be, it won't be a huge issue. Guys who start trying to pull lots of weight when they've never worked anything but those mirror muscles, I think they're the ones who end up having issues.
Current PRs at 242, raw w/ wraps- 525, 355, 605, 1485
Roboro tui, perimo vester adversarius
(Build yourself, destroy your enemy)