Not. Some rifle/mag combos don't work well with 30 rounds in the mag; it makes seating the mag too difficult.
Some are tougher than others, although I have always managed to smack them in*. Regardless, the GI mags that we get issued are loaded full, you are issued enough ammo to load them full, and IME if someone tries to explain that you can't, they are laughed out of the hooch.
The only.mil folks left in that I've see push the 28 rd thing was a First Army (slick sleeve) trainer at Ft. Dix in pre-mob training. He also said the the A2 bird cage wasn't a flash suppressor, that an M2 was only approved for anti-equipment use, and that we should practice trigger control of our M249's to get aimed 2rd bursts because "Grazing fire has never been proven effective". So yeah, wives tale.
*I would argue pretty strongly that if you can't seat a 30 rd mag with 30 in it on a closed bolt, something needs to be fixed. Slap it up pretty hard and it'll usually seat. Course I also seat full mags on a closed bolt PTR and CETME, something experts tell me is unpossible.