Picked evening - but to be more specific - when it's dark out.
In the summer it's a
tad bit cooler out and for me, it makes the overall mood a bit better. Plus traffic isn't so bad at night.
But most importantly, in my field of work, there are those that I've seen that have their incall in/near the more upscale areas of town - which is where the majority of my clients live. There are a few others that I've seen that are near friends/family so being out while it's dark out, makes it harder for prying eyes to recognize me.
Nothing worse than waiting at a designated spot during the daytime (while doing the 2 call system) and someone you know pulls up next to you and strikes up what could be a lengthy conversation. Or a call from someone saying, "hey was that you I just passed waiting in the parking lot?", that again could turn into a lengthy convo.
The only downside is sometimes it's a PITA to see apt complex entrances or building #'s.