I made a karo quilt a few weeks ago. Here is the thread. When i started to move the down around it tried to force it to where it looked like it would go. After getting a bit pissed i decided to just shake it a bit hang it up and see. Well in about 15min(of doing nothing) the down was very evenly spread.
For the gravitational effects on down well i have my quilt rolled up in my closet and it hasnt settled one bit since i put it in there 2 weeks ago.
The only way you would have issues with down shifting around would be if it was under filled. Since most if not all people here like to over fill i would say gravity and settling would have a minimal effect.