Mtnman,
I have worked on rocket sights for several different planes and I can tell you that the impact point is not in the center of the sight. I have spent a lot of time working with the various rockets that are available in game so I thought I'd share a few observations. I hope this jives with the trainers opinions,
1.) In the TA (or off-line with dive bomb-sight enabled) the green "X" is approximately where the rocket will hit. In particular if you slow down the film and observe very closely, the green "X" is where a rocket from the
left wing will hit, if you fire a rocket from the
right wing it will hit to the right of the "X".
2.) Rockets do not have convergence, they fly parallel to each other. At greater distance it looks like they come together but that is due to compounded deviation in the flight path.
3.) The rockets have different velocity's, so some of them drop more than others at a given distance.
As an example look at this sight I made for the Fw190F-8. You'll see that the two boxes are shifted slightly to the left and there's a target box for each wing.
I hope some of these points help you figure out the accuracy you're looking for.