Yes, I hung it before I cut it. However, the irregular hemline was immediately apparent so the problem was with cutting, not gravity.
No, I really don't think it's you, I think it's the totally normal thing all chiffon circle skirts do!
The "dropping" effect on the chiffon is absolutely instantaneous. All you have to do it pick up a ¼ or ½ circle by its short end and the curved part heads for the floor! You are supposed to leave it for a week because it does get a little worse as time goes on (actually this effect can carry on very gradually for years, especially if you wash the garment), but it doesn't really matter if you don't.
How much they drop depends on the actual fabric, and soft chiffons are worse than others, but they all do it without fail. Even stable wovens like a quilting fabric will do it to a small extent. Knits, too, if they don't have lycra to keep them in shape.
What I do to get nice straight hemlines is to cut my circles first and hang them somewhere by the short ends while I cut the rest. Then I make up the dress/skirt except for the hem. Then I put the garment on the dress form on a table, and measure up with a ruler to make a nice straight hemline. If you don't have a dress form, put it on a hanger and hang it in a doorway, or put it on a person and get them to stand on a chair or table (to save you crawling on the floor!) If you can allow a few days interval in the process even better, but most of the drop happens in the first 5 seconds!
When you measure, find the shortest/highest seam and work from there as that will be your minimum length. Does that make sense? You can remove length from the curves that have dropped, but not add it to the seams that haven't dropped. The only thing to be careful of is that because the
minimum length is determined by the straight side edges of the circles, it's worth making the sides of the circles a tiny bit longer than you need in case the cutting goes slightly off or there's a little puckering in the sewing.
You could also retrim and rehem DD's rehearsal tutu to the length you like in that way.
Good luck! It's really not you.