In terms of this hobby, almost all my sessions are between 10 am and 12 pm. Main reason is the fresh feeling and being the first client of the day (hopefully) and traffic is not really too bad at that time both to and from incalls.
Outside this hobby, there is no bad time to have sex.
Typically try to schedule most sessions in and around the lunch hour as that is the easiest time to get away from work for an hour or so. Also puts me in a fantastic mood for the rest of the day which I don't mind at all.