The official uniform policy used to say "Medals are worn only on formal occasions such as official uniform inspections, blue and gold banquets, troop courts of honor, etc." But now all I can find is a vague statement that "These medals can be displayed on the Scout's uniform at appropriate events." (http://www.scouting.org/CubScouts/Awards/Adults/other.aspx)
In our pack, we have always told our Cub Scouts that the medals should only be worn to the Annual Blue & Gold Banquet or if they attend a Boy Scout troop's Court of Honor ceremony. Personally, I think wearing them only once or twice a year makes them even more special.