There are a couple of factors at play here; clean up time, hair etc, which others have mentioned. I used to see a girl very regularly who was a real part time, infrequent escort, such that she only saw one guy on the few days she worked. This meant that she was fine taking cum anywhere, be it in her mouth, on her face or body, or the way the OP mentions, with open mouth, as she would shower afterwards. Alternatively if you are the last booking of the day the girl may give you more latitude to make a bit of a mess. Personally I find it quite difficult to give a facial without the risk of messing something up, so I tend to go for cumming directly into her mouth.