Girls that start there shift at 5-6 and you are the first client usually have a highter % of chances to be a better encounter. Usually it mean they work from 5 to 11-12, they still have time to party up after, they go back home around 4 am, they still have plenty of time to sleep and prepare the next day. Its really simple, more the hours advance, the more you can be on the receiving end of a "too drunk, too high" situation or a "bad client" result as well. No matter if she started her shift at 1 pm, if you book her at 8, she is at the end of her shift and the result will be similar as seeing her at 1 am...
For me early appointements (in term of shifts) always have numerous times proven to be better. Sure she can had sex with her boytoy, boyfriend or whatever, so they "fresh in term of sex" factor is not always true, but the fresh factor in term of state of mind is usually there.
Also when you book an early encounter, usually chances she get on time are much better. There is a lot of factors.
I personally go for these when possible and as Gentleman Jon its a stragey that proven to be effective.