If you are right-handed, you are most likely pulling the shots left. There are more reasons for this than just triggering. The ability to consistently dryfire a Glock without disturbing the sight picture in no way assures the ability to avoid pulling shots, especially under pressure or distraction.
I'm not claiming to be God's gift to windage, but I've shot a pretty good volume out of 8 different Glocks and I've never seen any indication that any of them shot left. Pretty much every time I make a bad call with no other influence, the shot goes left.
You can work thousands of hours on this, be a very good shooter, and still if you look carefully you will see an inclination to pull shots to the weak side shooting freestyle with a Glock. It may not rise to the level of being a problem, but it will probably be your error default.
My approach to this is always to center the rear sight. For elevation, I consider personal perception and mechanical tendencies (timing splits, driving the muzzle down, etc.). But for windage with a Glock, I think it's best to make the adjustments to the shooter. For starters, you need to be able to shoot reasonably fast splits with the sights tracking perfectly vertically.
Glocks cast shooters' errors in incredibly harsh light. I look at this as a gift. If takes some insight to be able to burn off fast splits on tight targets with a Glock. The pursuit of that goal teaches a lot about pistol shooting.
