Steve Voigt
Dave, I agree that the abutments should imprint a little less at the top. I don't think splitting is a concern, unless the fit is really terrible! But I think there's two other benefits.
First is that if the the fit changes through wood movement, the wedge tips might not be putting enough pressure on the iron, and we don't want that.
The other, which took me a long time to come to terms with, is that an absolutely perfect fit is not ideal, because the wedge tends to go thunk and seat without any significant travel. Having a slight bias at the bottom allows for enough travel between "barely engaged" and fully tight" to make the wedge easy to adjust. Of course that bias has to be really small to work right; it's all in the details…as I'm sure you've discovered.