When booking MC, does anyone know how seats get assigned to boarding groups? I've been on some recent flights where being in boarding group D or E can determine whether or not I wind up gate-checking my bag (I have a flight tonight where I've been stuck in E, which has me nervous).

I know First Class, MCS, etc get boarded first, and I can pay $20 for MCE if I thought it worthwhile, but I'd like to be among the first MC folks to board if there's a secret to it...