hi,
what i've done in the past is to test whether it needs weight or not.
i use masking tape to tape up the major parts together - body, wings, horizontal stablizer, etc etc. then i put a pen/pencil on the table and sit the plane on top of the pencil. make sure the pencil is at where the main gears are. you should be able to tell whether head is up or down. if it is very close to balance, i'd add some weight just to be on the safer side. you might also want to consider other parts, like interior and cockpit tub... but for commercial planes, you probably only have engines and gears left