Well Justin DID say, '"So help me God". And so did quite a few others.
What difference does it really make? If a person is honest and sincere, he/she will take the oath seriously; if not it wouldn't matter what he/she said or didn't say, he/she would disregard the oath once in office.
No difference at all. The record for honesty among those who add "so help me god" to their promises is not noticeably different from that of those who don't. But it gives Boomster and the Butthurt Brigade something to squeal about.