As they say, your mileage may vary. I am exactly on the other side, I don't remember the last time an upgrade *did* clear out of BOS at the airport (many have cleared in advance, but it is literally years for me since I cleared at the airport). Every time, the routine is the same: "first has checked in full, don't bother waiting". I board, not wanting to risk full bins and checking my bags, and regardless of where I was on the board, the upgrade does not clear. While obviously I can't prove that others have been upgraded ahead of me in many cases, there are a few cases where I did see the upgrades processed and observed that if the person wasn't standing right in front of the counter, they were skipped over on the list (and in a few cases not even called if someone else was standing there). Anecdotal, yes, but in my experience it does happen. I have also been on flights a few times where I boarded and then saw the flight go out with at least one empty seat in F. These kinds of experiences suggest to me that whomever the gate agent was couldn't be bothered to do the upgrades at the last minute (maybe they believed their own line that F was full because everyone checked in).

This may be partially because of the routes I fly (mostly BOS-LAX, BOS-SFO, and BOS-DFW). I would imagine ORD or MIA would be easier to upgrade, but these are all mostly pretty tough during peak times, so it is rare for more than one or two upgrades to happen at the airport.