Gary, I don't think that this is an issue with individual venues. From the tone of the Q&A on this subject, it sounds like FIRST wants the only banners in the field side of the arena to be the ones they post for their sponsors. It seems that the line of thinking is that by allowing teams to post banners, it somewhat cheapens the space that they have granted to sponsors, which is bad business for FIRST. In any case, just because a venue tells you that it's okay to post banners doesn't mean that it's okay with FIRST, and doesn't mean that you should do it.
Relevant Q&A:
http://forums.usfirst.org/showthread.php?t=1749