Lateral balance, although not as critical as fore and aft, is still necessary for good flight characteristics. If the plane is very much out of balance is will be apparent on the first flight. Balance your aircraft in both the fore and aft axis. The lateral balance is by far the easiest and takes the least amount of weight to correct. Be sure the wing is on as you correct the balance. Hobby shops usually have 6 oz. wieghts with sticky backs in 1/2 oz. segments. You can even clip the 1/2 oz. segments even smaller. Try it on the outside of the wing. When it is satisfactory you can put it under he covering. It will usually take a pretty large plane to require a complete 1/2 oz. On park flyers you can occasionally get by with a piece of tape.