The turn fin and the keel of the hull dig into the water in a turn and the prop wants to drag the boat transom to the left. So, the further back the rudder the better the leverage to control all those factors. I have found that for example on a mono I used to run with no trim tabs that the closer to the transom the rudder was located the more it pushed the bow down to the water. It tripped the boat more so than when it was further back from the transom just dragging along. Most boat setups use trim tabs to accomplish the SAW attitude though and the rudder further back for controlling the boat in the turns. I am sure someone else might have other ideas, as I am just speaking from what I have found on my deep vee boats. I have found that running the rudder further back from the prop also helps control prop walk on my gas boats with the bigger props. On my old nitro mono hulls with the smaller props the rudder could be located beside the prop and worked just fine..
John