The only issue I have run into with my square hole at the horn is when I got the anvil new, I radiused the edge near the horn and transitioned back to sharp about half way down the face.
Ended up if I had a tool in the hole, I could not use my biggest radius on the edge. I eventually radius most of the edge and mostly in the middle where it would not interfere with any tooling.
My rookie experience with a new anvil.