Re: Tenkan on piece of paper
I never thought that it made a difference whether you turned on the ball or heel or whatever of your foot, and perhaps that is your instructors point. Mitsunari Kanai Sensei has a video out that explains some really kinesiological aspects of various aikido movements, and one of the points that I took away from that video is that movement is initiated in the hips, and the feet will take care of themselves.
When I first started aikido, and attempted the tenkan motion, my thinking was firmly rooted in the positioning and motion of my feet. I equated the the foot rotation to the US Army's "rear march" from its parade drills. I now understand that this is erroneous.
By focusing the motion in the hips, you can easily control variations in the weight dispersement on the feet.