Brian's right. The "right" time to cut off a sacrifice branch/trunk chop, etc. is when it has finished doing the job it's being sacrificed for--is the trunk diameter big enough for what you want to do. etc.? That is the question--if it has done that job, then it's time--the work can be done on a chinese elm now, or into the summer. Early spring is the best, as that gives time to heal the wound before winter dormancy.