Side to side or lateral balancing is not mandatory like center of gravity balancing but it will help the plane fly and track better with less trim. I set up a hook on the ceiling and suspend the plane with a string between the tail wheel and prop hub on the engine. On kit built planes I do this before covering so I can easily make changes to the structure or hide added weight internally. On ARF's you can screw lead weights to the wing tip on many planes or open up the covering and glue it internally. If only a little weight is needed, you can put nails or pins into the wing tip.