My flakiness depends on external accountability. When I know that someone more anal than me is going to look down on me for my being late, then I am adamantly NOT flaky.
E.g. When I was on the rowing team in college for four years, practice began at 5am sharp and not a second later. Our coach was a major in the Marines. For every minute you were late, you had to do 50 push-ups; after five minutes, the punishment was jumping into the canal next to the shellhouse. This happened even in January, with corresponding water temperatures. I WAS NOT FLAKY; never had to jump in for being late.
However, if I feel that the person I am dealing with is just as flaky as me, I try to be pretty much on time, but I don't feel much grief if I am a bit late.
And if the person I am meeting is flakier than me by far, then I get pretty damn irritated with their behavior and lose trust in them. I deliberately choose to not work with them on important projects, and I put less and less stock in what they have to say as time goes on. And believe me, it takes a LOT of flakiness to get to that point, in my book!
