Here are the rules as I see them:

1. Small touring band gets middle slot from the locals as a courtesy. Hopefully it’s paid forward.
2. If it’s all local bands, whoever booked the show chooses the line up. Booking is such a fucking pain in the ass. If I email 15 venues over and over finding a night that works for them and I put a bill together emailing 50 bands to find the ones available and finding new ones after the first set inevitably flake you better bet your ass I reserve the right to take the middle of the night. Then again, I also use that as a bargaining chip when trying to book shows with bands above our weight. Or if another band becomes friends with us and I want to be cool to them.
3. Everyone agrees beforehand. No changes night of. That’s is, in the words of Santa/Crashmore, unprofessional bullshit.

For tiny bands, middle is almost unilaterally “best.” Everyone who came only for the opener is still there, those filtering in for the last band start showing up.