I definitely think it depends on how formal the situation is, even if you are close with the hosts. If it is very formal, I dont expect people to help clean up - but if it is pretty casual, I think a small amount of 'cleaning up' is appropriate, or at least offering. and by small amount, I mean taking your own paper plate to the trash if the hostess is busy putting food away, not like wiping down the table or doing dishes.
I dont want to thread hijack, but I am really glad this was posted, as I have been wondering about this very issue. I know it differs for different people, and the situation, and I dont think it is rude if a guest doesnt offer to help, but I had a situation and I am wondering if I am overreacting or if it makes sense to feel the way that I do. I'll post a spinoff...