Depends on who/where taught and the gear in use. Everyone (esp LE schools and agencies particularly) seems to have "the one true way." Been corrected on a variety of ranges for doing it "wrong."
Always put your mag in the pocket, that's where partial mags go.
Never do that, you don't know what position you'll be in / it'll drop to the bottom and be inaccessible.
Use your back pocket, not your front pocket. Use your front pocket, not your back pocket.
Always put your mag in the mag pouch, that's where you know to go for a mag.
Never do that, it's a "fine motor skill" / only full mags go in pouches.
Hold your mags between these fingers, not those.
Don't do it at all, it's a "fine motor skill" you'll fumble anyway.
Most seem to teach by process, not principles.
when you have a lull in a fight or opportunity to do so
rapidly exchange a partially expended mag for a full one
retain the partial in a place you'll know to retrieve it
Hand size and dexterity matters.
For uniforms, open top mag pouches are more viable for a return than flapped parade pouches.
Rate of return on the time some spend training it is also questionable. I think we have a thread here somewhere looking for examples of LE or CCW fights continuing after execution of a tactical reload.