well my weeping cherry gets to long all year so i cut it back all the time.i think its more relevent how you cut it as apposed to when.early spring/late winter is the best time if you can.if you cant and its a heavyish limb cut it about a foot away from wear you actualy want it cut which would be back to the nearest branch or bud etc .if it leans to one side cut it underneath a little then cut from above. this stops tearing.then go back when theres know weight on the limb and cut it back to the closest branch etc.if its small just clip it strate back to the nearset part thats growing.