I honestly did not attend any open houses before I applied, so I can't give you any advice on how helpful they actually are or when to go. I would think they would be more helpful if you are trying to narrow down which ones to apply to rather than seeing the ones you definitely are applying to.
However, if you are worried about travel costs and seeing programs that you may not be accepted to, I would only go to the open houses of schools that don't interview. During interviews, they usually give a tour and have current students as well as professors to answer any questions that you may have. As for the ones that don't interview, I wish I had gone to their open houses because when I got accepted, I had to rush to schedule to visit the campus. So those may be worthwhile to go to.