The key with back is strength. I have found a correlation between strong rowing strength and the ability to lockout a deadlift. I don't know if there is an advantage either way - sumo or conventional. I am guessing with upper back it's fairly even, though I may be wrong.
My advice is to not overthink it. Sometimes guys overthink the lower back so much that it form locks
them. Make your lower back strong. A little rounding might occur on heavy lifts, but I wouldn't encourage it. If you back rounds early in the lift it will be stressed the entire way.
I simply try to keep my back in a neutral position the entire lift, not forcefully arched, and not rounded. Keep your chest up, lead with the head and stand up. Not sure if this is helping much...
Here I am deadlifting 545 for 2 reps earlier this year. Notice my back position stays the same.
Trenton Bash 2 - YouTube
My advice is to build a brutally strong upper back, with lats capable of maintaining a contraction while holding an elephant off the ground. If you build amazingly strong lats you can help to maintain a constant shoulder position throughout the lift, will resist your shoulders slouching forward, which will help to keep the lower back more neutral.
I thought my upper back was strong until I really started to push myself. I currently strict Pendlay row 365 x 5 reps, and have hit dumbbell rows of 215 pounds for 10 reps:
215 Pound Dumbbell Rows - YouTube
Upper back rounding is not necessarily bad but it will change the mechanics of the lift forcing you to pull more with the lower back, and this may lead to lower back rounding.
Am I making any sense here?
Matt Kroc also believes that heavy rows helps with deadlift lockout, so at least I have one partner in crime. At one point a few years ago my rack pulls were pathetic compared to my deadlift, until I upper my rowing strength.
EliteFTS.com - 225x25 Kroc Row - YouTube