Bednobs
Every time I use it, I book.
The tickets are transferable to a different bus-time, so I can see no reason not to book - they use the booking information to plan their capacity-loading.
What I have observed at Reading Station is people in the office trying to book a instantaneous seat and being told that the next bus is full - so they will have to wait to see if those booked do actually show. Hence I believe best to book.