I have found that, for me, rounded lips are more forgiving and easier to tune and fish on deeper diving lures than square bills. I think maybe it's because deeper divers are more difficult to get right in general than shallow running square bill cranks, so I find rounding the lip makes that one less thing to worry about. And I've found that a square bill will flip up a bait to vertical more consistently when it's burned into a piece of wood, so it's hooks are thrown up and over the wood, where a rounded bill can roll the bait more to its side, allowing the hooks to snag.