My understanding is 864 is required at the time of interview.
Scenario # 1
USC is sponsering relatives who are outside US.
this is becz CP process, and I believe 130 form have no instruction to send 864 along with.
Scenario #2
USC is sponserign relatives who are present in US.
One of person I know who filed and 864 is only asked at the time of interview. his attorney never filed 864 before.
Btw many ppl shared even Manadatory doc were not sent along with applicaiton and they accepted it and later sent RFE to them
I belive in family bases cases almost all receive interview call and 864 is only reqiured at that time. correct my understanding, if I am not aware of somthing.
Good Luck