Usually night time, as that is what is most convenient for me.
There is a good number of prossies I've really wanted to see, but never managed to as their daytime hours just don't work for me, especially the more popular ones with limited availability. I can push things around or take a day off work to see them, but it is tricky for me.
I generally prefer doing most things at night time anyway, as less people about, and I'm not a fan of crowds. I'm the sort of weird guy who goes to do his food shop at 3am at a 24 hour supermarket...
I have had a fair few punts in the morning or afternoon though, usually with the more popular indie girls, who tend to stop being available after about 6 or 7pm.
I've had good and bad punts at all the different times of day though. A shower and a nights rest doesn't wash away all the sins of a prossie who doesn't take care of her hygiene properly, or who just doesn't care about giving good service, or who works overly long hours seeing too many punters to give a non rushed service.