Doesn't B6 assign passengers on basic economy fares from the back of the plane toward the front (otherwise a not-full flight would end up imbalanced)?
It does seem that's the biggest risk of having someone sit in the middle seat (involuntarily) if you're in the rear. Otherwise if the plane starts to fill up and there are only middles available then a late booker that does have seat selection would probably choose a middle closer to the front than the rear. So on a half-empty flight, I'd probably keep the seats up front and hope the middles stay empty.