Tweaking mechanics is a constant in professional sports. That's what makes them professionals and why they continue to improve with propher coaching. Golfers, Pitchers, QB's, whatever the sport doesn't matter, they see things that players can't see.
I wouldn't be too concerned as a Giants fan about Manning as much as I would about Coughlin.
As a Cowboys fan, this doesn't matter as. The Cowboys will win the East with Peyton Qb'ing the Giants.