For sure it happens in ATL, maybe usually not so blatantly, but certainly the holding back of UGs so that everyone has boarded and then letting their pals board in F. Yeah yeah, you board you lose. The spirit of that "rule" is to give the GA some leeway to get the flight out on time ahead of meticulously following the UG list in cases where a delay might ensue. It's not so that NRSAs can score UGs at our expense.
Then there are GAs who make a point of following procedure and clearing the UGs before boarding ST+ despite the presence of NRSAs or coming onboard and bringing you up-front.
It's probably a controversial point among employees: those who think 'wth DL screws us with this so why should we not do what we can for ourselves' and those who think policies mean something and should be upheld. We see the same attitude on this board: those who take advantage of the glitch that allows people to "change" to the same flight if it's delayed and get on the UG list ahead of their turn, and those who think it's pretty despicable.
These things are usually a zero-sum game and those who don't play by the rules (in their spirit as well as their letter) usually ruin it for the rest of us.