Good question. Actually, I can't seem to find it published anywhere but I've never had a problem doing this at any airport I go to when flying AS.

Not once have I been told to go to a different line or "why are you using this line?" I've also never checked in at LAX. I avoid that airport like the plague.

You could politely remind her that it's a courtesy extended by NW to AS elites during check-in (to which she'll promptly tell you to eat dirt and move to another line probably - if she's that mean). But it's worth a try. You can also say that you don't have a problem checking in through the MVP/elite line at most every other airport with no issue.

BTW...I've done this at Las Vegas when the "normal" check-in line was literally 100+ people long. I was #1 in the elite check-in line. Not a blink of an eye or anything and the check-in agent was clearly having a bad day.

-RM