18' sounds like a long span to me for 2x8's if they are 16" on center. I'm not for sure though. According to what you are saying I would agree. Since the header is a load bearing part of the wall, but in your case you are saying it's not load bearing wall. Probably better to just agree with the inspector and get it done the way he or she wants since it's not that much work anyway to install a header. Probably more of a hassle to argue and prove your point than to just do it is what i'm saying.