I've seen pictures of Ford and W. Bush with their feet propped up on the desk, but if you look they were careful to have their sox on the desk not the soles of the shoe.
I thought I saw the Reagan pic posted here last time this came up, but it must have been the Bush pic. I googled for it, but all I saw was Reagan eating at the desk. Obama’s posture is different, but maybe because he’s taller.Not that I believe he has any respect for the office either.