Always when it is less busy, combined when it's the best time for you, LOL! You also want to consider the time of day, given the neighborhood, safety, parking, and how discrete you can be. Very situational.
Time of day should not impact selection, other than how busy the place is. I think there are times to avoid, like Friday and Saturday nights. Sunday mornings can be good, but the girls can be very tired then, depending upon the amount of business the night before.