Here's the issue y'all are missing, and its not unique to Glock.
If you cram that last round in the magazine, its gonna be a mutha to seat that mag with the slide closed.
For me, I don't lose sleep over that last round.
Load the mags down 1.
Insert the mag (slide open or closed).
Operate the slide to load the top round into the chamber.
The mag is down 2 now.
Load one more round into the mag.
Insert the now-down-1 mag into the pistol.
You're loaded now.
As for your spares, you want them loaded down 1 anyway. Ever heard of a tactical reload?
You know, where you've fired a buncha rounds, and you know the mag in the gun is getting short.
You don't want to have to be hammering a loaded-to-the-absolute-max magazine into your closed-on-a-live round slide during a firefight.
How much would it suck to fire the first round after a tactical reload, only to have the not-completely-engaged magazine to fall into the darkness on the recoil of that first shot.
Firearm magazines are like the gas-tank on your car. Don't try to cram the last little bit into the tank. All sorts of bad things can happen, and whatever "benefit" it gives will never be worth the risks.