I could see them distributing people evenly across the plane when the flight isn't very full, however my experience in flying Spirit on mostly-full flights has been that they assign seats starting in the back. That way they leave the more expensive pay-for-your-seat seats until the end in case they can upsell people last-minute.

Since I'm always flying with my partner and we'd like to sit together (though we're fine if we aren't), this is my trick for increasing my chances of getting seats near the front:
Starting at T-24 hours, I log onto Spirit and start to book the same flight so that I can get a look at the current seatmap and which seats have been assigned. I then repeat that every hour or two. As more and more people check in, the available seats start to dwindle. Since the AI tries its best to seat people on the same reservation together, I wait until there are only a few pairs of adjacent seats remaining. That's the point when I log on and check us in for the flight.
Since I started doing this, we've always had seats in the first 1/3 of the plane.

-Rob