You can rough center it, but you still need to shoot it and fine tune it once its installed.
A sight pusher really does make it much easier, as the tool allows for more precise adjustments, and if youre installing night sights, its a must have.
All my Glocks have the rear sight to the right side, where the edge of the sights base is even with the edge of the dovetail. Its not actually "'centered". If I center them, they all shoot to the left a good bit. You will notice if they are off, and even just a little bit.
Be aware, if youre removing the factory plastic rear sight, its probably going to be trashed when youre done. I've done two so far, and even when pushed out with a pusher, the plastic sight body was destroyed and the metal retainer had to be driven out with a punch.