I recently asked one WG if I could book in advance, she said she only does on the day bookings as she has been messed around too often with advance bookings not turning up. I suspect that she also didn't want to commit to bookings until she knew how she felt on the day. For example, if she has a late night booking, she often doesn't start until late the next day. In her shoes, if I turned down a late-night booking in favour of a booking the following morning who failed to show, I would be a bit pissed off. However, not being able to book in advance can be a pain.