Last weekend for ‘Assassins’ at Off Center Stage In this thought-provoking musical comedy, characters include nine assassins who have attempted to kill the president of the United States. The story examines how and why each of these outsiders can stop and alter history. Performance today at 2 p.m. at Off Center Stage, 314 Richardson St. in Grass Valley. For information, visit www.SierraStages.org. InConcert Sierra features Stefan Jackiw today in  Grass Valley Stefan Jackiw made his professional debut at the age of 12, when Boston Pops director Keith Lockhart invited him to perform on Opening Night in 1997. In 2000, he …