i have left mags loaded for several years without issue. The issue with loading full or less is based on this in my experience:
can you and does your weapon allow you to easily seat/lock a fully loaded mag with a closed bolt for a tac reload in a fight.
stacking tolerances are a thing as well as technique and shooter strength. A mag should not require a slap to seat. In fact it is a bad habit.
If you cannot, download the mag. With the AR, download two. The top round n the right of the mag is an even number. Creating the habit of mags being loaded the same. This allows for chamber verification during the admin loading process without using the stupid FA, by quickly removing the mag after chambering and seeing the top round now on the left.
Pmags will work with 30 in a 30. I still run 28 in them. But proof the aluminum Gi mags may not. Have round in the chamber and insert a 30 under the closed bolt and shoot. Stoppages are frequent here.
We ran 28 in our 30s. And some ran 26 due to the heavy res mag springs we ran in 933s due to the fast cyclic rate.
for a fighting tool, no matter the weapon, proof you weapon with the mags to be deployed and the ammo to be used. Its a Traid. Trust your life to nothing that has not been proofed