Do I need to check a box to show my sig for each post? Is there a default setting somewhere I can set to show it automatically, for new threads and replies, every time unless I specify otherwise?
Yes. No.
And you also have to re-check the box if you use preview, because preview throws away your signature. 
Wow. Sucks.
I believe there’s a policy that each poster include the sig only once per thread, anyway.
Yes, we prefer that you only use your sig once per page of any thread. Please.
Is this an official rule? It’s the strangest thing I’ve ever come across on a message board. 
Naah, we used to ask that because bandwidth was limited and we wanted to squeeze every molecule. Now we just think once per page is enough.