I have that annoying Virgo trait of being able to walk into a home for the first time and notice that the picture to the left of the fireplace is 1/8-inch lower than the equally sized picture on the right. My first shots at the range are usually in the ten ring, if not bull's-eye. I tend to see straight the first time.
That said, if you're new to Glocks, the Glock trigger takes some getting used to. Slip a laser on that gun and have an observer watch the projected dot while you go through the firing sequence. Many are astonished at the movements their hands actually describe on the target. Dry-fire practice with a laser fast-tracks a new shooter's learning curve.
If you are a new shooter with a Glock, start with the rear sights centered (I'm assuming an indoor range), and work on your technique. Sights should not be adjusted to compensate for poor technique. The fixed sights on our G19 and G21 are centered, and I shoot them pretty well.
All of that said, the rear sight on my dedicated target gun--a G17 with DP FO/Adj Sights--is tweaked about one mm +/- to the left, as snipe1 says.