Not necessary 99% of the time, but convenient.
Lee Precision's thoughts on the matter can be found in these threads:
One thread contains a lively discussion of the FCD and the function of the post-sizing carbide ring in the FCD. The phrase "9mm" is in the thread title, but don't let that dissuade you. It contains responses direct from Lee Precision, too. Read the whole thing. It is worth it.
http://thefiringline.com/forums/showthread.php?t=465091
this thread contains a poll
http://thefiringline.com/forums/showthread.php?t=465603
Lost Sheep