I suppose it's possible the shaft could still be flexing from the forces of the shot at 10 yards, and would be flexing minimally, if at all, at 20 yards. Assuming you are tuning with a truly bare shaft, the results at 20 yards should give you a better indication than the results at 10.
When I bare shaft tune, I like to do it with feathers mounted to the shaft that I have cut back to the quill, so that my tuning takes into account the weight of the quill and glue on the rear of the shaft. I have to be careful when I do that, because the quill alone will stabilize the arrow after some distance, provided it is close to being tuned for the bow. In my case, 10 yards might give me a better indication than 20.