I recall a flight many years ago (I believe on a 737) that was only about half full. Seats were not assigned, but we as SLF found the first 15 rows or so taped off as not available. The cabin crew when asked indicated that the reason for having us all in the back half was balance management. I also recall a ride in a small private airplane with a friend where his walk around included manually testing the tail-dragger weight by trying to lift the aft fuselage off the ground. Finding it too heavy he re-positioned a rather large toolbox from the aft end of the cabin to under my feet at seat 0B to get the balance right. Good lessons that there cannot be any shortcuts when it comes to maintaining weight/CG within the design limits!