I know the comparison isn't exactly right, but think of it this way: if it was bad manners to only write one version of an AU, then we would not have as many "here they are! on a bus! there's probably porn!" stories out there. Just write what you wanna write. I know for a fact one of my flisters is writing a film noir AU, and I am too (in theory. in that, I haven't started yet), but the pairings and bands are totally different, so pffffft.