I've left magazines laoded for many years, a few as long as 25 or so. They all seem to work fine.
When I messed with AR's years ago, I did notice it was easier to seat a magazine that was downloaded by a round or two when the bolt was closed. It helped also when hitting the bolt release to close the bolt on a fresh magazine. The gun would sometimes either close very slowly when cold and/or dirty, and a few times stopped. The bolt could be closed by pushing it shut with a thumb in the cutout on the side (pre-forward assist on commercial Colt AR's). Dowloading by a round or two helped noticably.
"Far better it is to dare mighty things, to win glorious triumphs even though checkered by failure, than to rank with those poor spirits who neither enjoy nor suffer much because they live in the gray twilight that knows neither victory nor defeat." -Theodore Roosevelt-