What Ken said and with final sanding done when the tip is attached to the wing. Best if an easy to sand glue is used such as a wood glue like Tite Bond II or Elmers (Aliphatic Resins) and not CA or epoxy. Apply glue and clamp in place until dry. You can even glue the balsa block in place and then rough out the shape with razor plane before starting to sand.
http://www2.towerhobbies.com/cgi-bin...&I=LXAA64&P=ML
Very handy tool for a builder, and inexpensive.