If its one of my Glocks, I usually am inserting a fresh mag with sufficient force to drop the slide. I rarely ever shoot to slide lock when competing in USPSA matches in Production Division. I shoot based on the way the COF is laid out, and plan my reloads.
With my STI, I pinch the rear of the slide as I was taught that way many years ago. I do the Glocks the same way when I initially load them prior to beginning a course of fire. I've not had an opportunity yet to attend a Defensive or Combat Handgun course, as I expect the weak-hand over the top is what is taught.
Many of the folks I shoot with use the weak-hand over the top, but I've seen lots of shooters come close to sweeping their weak hand in front of the muzzle when attempting to do that. Most of the LEO guys and some of the guys who have attended formal training use the weak-hand over the top method for stoppage clearance/reloading.
I usually make it a point to keep my parts away from the dangerous end.
I know several guys (RH) that use their weak-hand thumb to drop the slide via the slide release after a reload so they don't break their strong-hand grip.
On a side note, here is a link to a video of Travis Tomasie (he shoots with the AMU now) practicing reloading an Open racegun.
http://www.doublealpha.biz/images/travis_reload.mpg