Yes, actually all of those former officeholders keep their highest formal titles according to convention. For example, in the NYTimes the usual treatment is to refer to them by that title at first mention, then Mr. or Ms. (or Mrs. or Miss if specifically requested) later in the story. That is the same way they handle the titles of current officeholders.
Personally, I don’t like it. I’d like to keep honorary titles to an absolute minimum in our republic, but Romney would likewise be referred to Gov. Romney. That’s the way we do it.
As long as it’s proper. :)
As I said, I’ve never seen somebody, who used to have a political position being referred to as though they currently still held it.
It’s cool, carry on.