I use the push-pull method. When I grab a magazine out of a mag carrier, my hand is around the body of the mag. It would be more movement (and therefore more time) to slap the bottom, so I just give it a good push and then pull to make sure that it is seated. Then press the bolt release unless it is a combat reload.
The only time I slap the bottom of my rifle mag is if I have a malfunction, or if I pull down on it and it has failed to seat properly.
Either way is correct, and if someone criticizes you for using the push-pull as opposed to slapping the bottom (or vice versa), well, I would not take much more advice from them.