It really depends. The back round can allow for some greater poundage do to some leverage factors but the flat, locked back is the ticket for health and longevity at this stuff.
Just as an aside...OL'ers tend not to round their backs much if at all when performing 1RM dl's. Not sure if this is a carry over from the OL back squat and the general upright torso in the receiving position of the snatch and C&J or what.
"Survival will be neither to the strongest of the species, nor to the most intelligent, but to those most adaptable to change."