I just automatically ask if I can bring anything. I'd rather ask and be told not to bring anything than not ask and have someone annoyed because I didn't. Mind you, I have a friend who was hosting a baby shower and got soooooo angry at everyone offering to bring a plate because the invitation clearly stated SHE was hosting

More often than not when DF and I are hosting, we just tell people to byo booze or anything specific they may want (whether that be food or drink) and we'll supply everything else. For most things in our group though, regardless of who is hosting, everyone will still bring a plate - one person will do a green salad, one will do a pasta salad, etc. Even when we tell people not to worry, they still like to bring something.