You do need to have the plane assembled to do the final balance. I use one of two ways to get around the problem of weight on the outside. I either cover everything except the top of the wing before balancing, or complete the covering then drill a hole in the wing tip to insert weight into, then put a small patch over the hole. Solder works very well. The weight of the top covering has never changed the balance by itself, so that is the way I prefer. I use the second method on solid (foam) wings.

Some attention to balancing as you are building is necessary too. I weigh ribs and try to make sure that the heavy ones are inboard and evenly split between left and right halves of the wing, for instance. Same for sheeting. This saves having to use more weight than is absolutely necessary.

Someone, a very long time ago, told me that every screw, nut and bolt should be in place before covering, then removed, so that you are not installing things like horns after the covering is complete. It's a PITA, but, has saved me countless holes punched in a good covering job.