I went to RSA in April. Here is what I learned.
1) CBP in Spokane would NOT list the suppressor on the 4457. He said check as accessory, with copy of stamp.
2) The ATF regulations list things like machine guns, destructive devices, etc, requiring a 5320.20, and do not list suppressors as being required, but also did not affirmatively exclude them. I electronically filed for ATF 5320.20 for my suppressor to be safe. It was free and fairly quick.
3) In ATL, I asked the officer checking us. He said all they care about is your tax stamp, to prove you are not bringing home a foreign suppressor. Of course, he does not work for ATF.
On my next trip, I will do the 5320.20 again, unless wording changes so I'm positive I don't need it. If I was in a quick travel scenario, and it wasn't going to be done on time, I would not be afraid to travel with the can as an accessory in the gun case, with copy of tax stamp rubber banded around it.