In the 1970s, the Army taught us to only load 18rnds in the 20rnd mag and only 28 in the 30rnd one. This was because of concerns about "reliability".
From a casual shooting perspective this is not a problem.
From a military perspective it is a little different. In operations magazines will get filled with dirt and dust. If that works it's way into area between the first round and the feed lips it can lock it up tighter than drum. I have seen this many times when the soldier tries to chamber or reload and can not and then bangs the weapon on the ground to get it to load.
Putting fewer rounds in the magazine can help by reducing the tension on the spring. On the other hand this will cause the magazine to wear out slight faster due to the extra play in the magazine.
Most pros don't notice this problem because on a regular basis they unload their magazines and clean the mag and the rounds and then reload.