@neriah, I also prefer the flat surface for engrosser's. I should clarify, I think the habit of using a flat surface today could be from when we were taught cursive writing in school. For me, writing engrosser's on a flat surface may simply be from my early years in school holding a pencil, so writing, even with a pointed pen just feels "correct" by using a flat surface.
I also agree that many hands can be done on a variety of slopes, some better for posture, some for paper rotation, some for thinner inks. I think everyone has answered with some great points.