Prebooks are important, would you go into work if you knew there was a chance you might not get paid for all of your hours?
When you have a $500 or more overhead every trip you need to make sure you will make that back then make sure you will make enough profit to justify the stress and time put into making a trip.
I won't make a trip unless I have enough prebooks lined up to cover everything and make it worth the trip even if one or 2 cancel. That's why I do prebook and multi hour specials it helps me stay organized, have a sense of security, and stay low volume.