Personal opinion: a reason I find crossposts annoying is that I always feel I have to click through to the crosspost and see what the commenters are posting on the crosspost as well as the original post. That's a potential downside to think about when considering a crosspost. Obviously crossposts do have advantages too! I personally am not going to trash a crosspost just for being a crosspost.

I'll also echo RobinZ's suggestion of adding from/to notices whenever you do decide to crosspost something.