Not only should the shelf be radiused but the sight window should be also. Whether the shelf and riser have been contoured this way or if you do it by applying something like a toothpick doesn’t seem to matter. You just simply need to have that single clearly defined pressure point. You will achieve greater forgiveness if both the high point on the shelf and on the sight window coincide with each other and they are located above the throat of the grip. (Assuming a contoured grip.)
The single defined pressure point assists in initiating the oscillations of going through paradox in the arrow with no “funny stuff” happening. This is hard enough without throwing in weird arrow flight to boot.
I have no actual empirical testing to support this. HOWEVER, I have seen the results of others attempting to shoot flat shelves and sight windows. Really bad arrow flight for one thing. And the “evidence” shows up as interesting wear patterns in whatever they may be using as shelf material. You’ll see 2 or more wear points along the shelf and sight window. The arrows seem to be either be striking the shelf and riser multiple times during launch, or upon release they are not always being pressed against the riser in the same place on every shot.
I have seen some who accidentally created a “work around” by running extremely high or extremely low nock points. This gets the arrow positioned on the shelf when at draw such that the arrow launches from either the back edge or belly edge of the shelf, basically inadvertently creating that single defined pressure point. Such an arrangement is still very much less than optimal though in that it makes the bow more likely to still randomly screw with arrow flight due to spurious bow torque that the shooter may accidentally impart to the riser.
