Hopefully, somebody can put me out of my misery and direct me to the official source of this information.
Unless this is in a technical service bulletin, the only reference I can find is the end of Chapter J in the section "Hub Unit - to fit" where step one states, "Position the hub on the stub axle and fit the hardened key washer and a new
shrouded nut." (added emphasis on new is mine).
This is one of those situations where I would be willing to believe that Crewe was indulging in an excess of caution, but someone who is in the fasteners business and/or a metallurgist should be consulted to confirm this. Bill Coburn's statement, "it conceivably distorts the threads," is a conjecture, and everything else I already know about threaded fasteners indicates if something is tightened beyond its elastic limits the threads on not only the fastener, but the thing it threads on to, should both be distorted such that they should not be reused for fear of stress-induced failure. If they have not been, and one has to presume that a correctly tightened fastener has not been tightened past its threads and the the thing-it-threads-on-to's threads elastic limits both should be fine for reuse.
So, I've now found the single word reference to "new" sans any dire warnings (that I can find, anyway) in the official documentation and my thinking aloud about why and how one might go about questioning it in a manner that, with sufficient care and research, should yield an answer regarding whether this "new" is essential or a matter of exercising extra caution.
P.S. Posting direct links to the RROC-US forums here is often an exercise in frustration for this forum's members unless they also happen to be members of the RROC-US and able to log in to the RROC-US forums (or are already logged in) when the link is clicked.