It is not unusual for this question to be found on general firearm forums, but those dedicated primarily to Glock handguns as well.
Some express concerns whether or not the stamped steel slide stop lever will eventually round off and no longer retain the slide after the last shot is fired if they use it to release the slide after inserting a loaded magazine. Others worry that the thin slide stop lever might round the rear vertical edge of the notch cut in the slide for hold-open after the last round is fired.
In some of these threads, the original question may actually be answered but in many instances it becomes very secondary to generally boorish behavior such as crude remarks, personal attacks and name-calling.
I thought it might be novel to attempt a sensible approach and check with the manufacturer. Looking on page 23 of Glock’s “Instructions for Use” (REV.11/08) manual under “Loading and Firing”, we see that Instruction 4 states: “After the last round has been fired, the slide remains open. Remove the empty magazine from the weapon by pushing the magazine catch (19). Insert and new magazine and then either push the slide stop lever (27) downward (see photo 2), or pull the slide slightly backward and allow it to spring forward. The weapon is now again ready to fire.” (Bold added for emphasis.)
My goodness gracious alive! Either method can be used. It seems that the shooter has a choice and will not be struck blind and dumb if he or she chooses either method...at least not according the pistol's actual manufacturer.
Someone frequently brings up the concept of fine motor skill loss under stress and suggests that retracting and releasing the much-larger slide is more easily accomplished than depressing the smallish slide stop lever. On one site this suggestion was countered with the claim that a very well-known competition shooter uses the slide stop exclusively to drop his Glocks’ slides, and reports no damage.
My opinion (and that’s all it is; feel free to disagree) is that anytime a steel part rubs against another steel part a tiny amount of wear occurs. For something to never wear out, it must never be used. It strikes me that despite the company’s claim of “perfection”, there will also be miniscule wear on the Glock slide stop lever if it is used to release the slide. The keyword here is miniscule. For those shooting their Glocks but moderately, it will probably never be an issue. Based on my own observations and conversations with “heavy duty” Glock users, it is quite unlikely that the practice will damage the lever (or slide for that matter). If it does, the part is quite inexpensive and very simple to install yourself.
Pick the method that works best for you. One requires but a single hand to accomplish but might induce a tiny bit of inconsequential wear to an inexpensive part that will not cause the pistol to “jam” or otherwise suffer a stoppage. The other requires the use of two hands, but both hands are already in close proximity if a fresh magazine has just been inserted and it is unlikely to be injured in the time taken after seating the new magazine to pulling the slide rearward and letting go.
That’s as honest, accurate and civil an answer as I can provide.