You don't have a choice in the theater. I thought of complaining to the guy in the projection booth, but you know, they're kind of stuck with what they're given.
EDIT: Actually, I do watch Mononoke with the dub, despite Claire Daine's awful voicework for San, but that's because for some reason the music volume and voice volume on the Japanese audio track is really screwed up, so the voice's are really loud and the music is hard to hear. It's possible that's just my DVD though.