After discussions with the IEEE copyright office, papers are now checked for similarity once, immediately after the track review paper submission deadline. There is no need to re-check after acceptance since it seems rather unlikely that an author will submit an original manuscript and then submit a plagiarized version after acceptance of his original work.
This approach also avoids author needlessly trying to lower their similarity score for their original papers. This is not the goal of similarity checkers - they are only meant to prevent plagiarism. Since an author seems unlikely to inadvertently copy somebody else's paper, there is no need for honest authors to confirm that their paper is indeed their original creation.
This change avoids bothering chairs with lots of extra email and reduces the cost of plagiarism checking for IEEE, as they pay for each submission to the plagiarism tool (iThenticate).
Authors who wish to check their paper can use the Docoloc service directly by creating their own personal account.