I usually retweet things that inspire, amuse me or just make me think. Just because something amuses me or makes me think does not mean I agree with it or find it very bright. I think that is a problem people have with relating to etiquette, they think they are rules set in stone for everyone instead of guidelines. At least that has been my experience. I do think that most people would hold your friends view on the matter mostly because people tend to spread what they like unless they put a disclaimer of "ZOMG How stupid!!" attached to it. They have to add their point of view to the matter instead of letting it pass on through them without bias being attached.