Here we go again.
I just stuff dem rounds in da mags 'til da mag don't hold no more.
Sorry, charleseast, I couldn't help myself. This question comes up every few weeks. You'll get some people who load all their mags to capacity; 20 in 20-rounders, 30 in 30-rounders. I'm very happy this works for them.
You'll then get responses from people who load 20 in their 20-rounders but download their 30-rounders to 28 rounds. These people have typically been taught this practice by guys who have "been there and done that." Some of these people also may have first hand experience with full 30-rounders not seating on a closed bolt without really smacking the crap out of the bottom of the mag.
If you're shooting is limited to punching paper from a bench, stuff you mags until they won't hold anymore. If you are using your AR for matches or for work (i.e. LE or military), you may be better served with a downloaded mag. Figure out what works best for YOU in YOUR situations and go from there.