HOW IMPORTANT ARE "D" SHAPED RUNGS?
As the ladder is leaned against the wall on a 45degree lean the rungs should then become a place of comfort for the time you are on the ladder.
D rung ladders give you nearly 2" of flat surface for your feet to rest on while the Square rung ladder gives you a corner to stand on. So imagine all your body weight standing on the corner of a square. So for comfort and a more secure footing, the D rung is better then the Square Rung in our opinion.
Picture on left is our Ladder showing "D Rung" picture on right is a major supplier showing Square Rung.