For me, I load magazines to capacity and then check if they still have a bit of give (so I could almost seat another round). If they have that give, I keep them full. If not, I download by one.
I do this so that under stress, I can more easily seat a mag on a closed bolt.
At my last Gunsite class (7 years ago), I was in the final shoot off at the end of the week. I had made it through two elimination shoots already and was in the last. The shoot was draw, engage three small targets, perform a reload, and shoot my side of a split metal popper, and be faster than your opponent.
I was ahead and got to the popper. My pistol still had rounds, so as before, the slide was forward. I swiftly dropped the mag and reinserted the fresh mag, aimed, and fired quickly at the popper. Miss. No problem, opponent was just starting his mag change. Slowed down, focused on the sight, and pressed the trigger….click! Immediately went to do a tap-rack-fire when the magazine fell out of the pistol onto the ground. I immediately went for a new mag and just as I was seating it, “bang”…opponent dropped the popper. I finished the reload, and dropped my popper…and lost.
I had just gone through 4 1/2 days of quality training and done many many mag changes on full mags. Never had the issue until it was under the pressure of winning the silver raven pin. Thought about that the whole rest of the day and the drive back to San Antonio from North AZ.
If it is a mag for SD/competition, and if the mag is very tight when fully loaded, I download by one.
YMMV