A) I think it depends on the situation - if it's a good friend, I'll ask what I can bring, since I know they'll tell me if they want me to or not. If it's someone I don't know well, I usually wouldn't bring anything unsolicited unless it was more of a hostess-gifty thing for a housewarming or something, rather than something FOR the party. Unless there is a fun theme and I can't resist. And if it's a dinner kind of party, and they are omnivorous folks, I sometimes offer to bring veggie-friendly food.

B) I usually plan to have everything, but if someone asks ahead of time and they can be useful, I'll usually take them up on it. It's nice when people bring things unexpectedly, though, if it's an informal kind of thing.