I can't say anything about the flutter, my crystal ball has a big crack in it and I ran out of CA!! Can't even fix the thing today and I may need it later?
I got in the habit of covering in one piece and I fill all the gaps, top and bottom. Before I started doing it that way I flew without sealing and never had any problems that i knew about. Then my advanced instructor mentioned sealing them to me and one way or another I have been doing it ever sence. I keep a roll of clear Ultracote on hand just for gap sealing, I even use it on planes that are glassed and painted. On these planes I can remove the covering without removing the paint just by using my heat gun, much better then using tape. Only real way of knowing if you are going to have flutter is to fly the plane and see for yourself.