You don't have to provide any bookings - the guidance notes for ECOs has some comments about this because they know you can't make a confirmed booking without both people actually in the UK. We didn't provide any bookings at all or even a provisional date (only a month...) and I know others who did the same.
Really it's down to whether the ECO believes you with regards to your relationship or not. A provisional booking with a hotel that you could cancel anyway doesn't actually prove a wedding will take place.