I am a little confused. I have never heard of calling and making a request being a bad idea. I do know that some guys prefer not to do so. If that is their preference, then that is best for them. For several years now, I would not consider doing otherwise.
When I started I read a lot of reviews of each girl to form an opinion of who would be good for me. Then I always made a reservation. A particular girl could be booked, off for the day, starting later in the day, on vacation, etc.
I see very few now and have known them long enough that I call direct. The important part is that I call. Because I don't want to be disappointed.
I might add that there is no absolute guarantee that the girl will be available. Sometimes there is a mixup or for some other reason she is not available at that exact time. Then you have the option of choosing a Plan B or waiting for another day. I have had that happen, but rarely. With those that I see regularly for the last few years, we know each other well enough that I am cool with waiting. Believe me, that attitude will take you a long way when you do get her in the room.
Torito