I went on a long cross country flight once with the autopilot turned on. There were scattered clouds that caused up and down drafts. The D-180 was constantly asking me to adjust the trim so the pitch servo would not have to work so hard. So I turned the pitch function off and let the plane go up and down with the drafts. There was no more climbing or descending trying to hold a constant altitude. It made for a much more pleasant flight. If the average altitude eventually became too high or low, I would adjust the throttle slightly.