I also understand that David Broder wrote a book in 1967 which claims that George Romney marched with MLK.
We're in the realm of "proving a negative" here, and the principals are all dead. The essential point is that Mitt comes from a family that has a long history of taking decent stands on civil rights. There's not much need for controversy on this point.
The little lies matter because they tear the veil obscuring the one big lie that is Romney’s campaign. Posing as a conservative after a career as a liberal politician in a socialist state uses up all Romney’s reserves of credibility. He’s got nothing left to protect him when he stretches the truth along the campaign trail.
The latest little white lie will hurt. They will all hurt, and eventually one of them will kill his candidacy.